Mr. Rodriguez’s infraction was open to debate: Rounding the bases in Toronto, with his team leading late in a close game, he appeared to yell something at Blue Jays infielder Howie Clark—“Mine!” or, in Mr. Rodriguez’s own version, “Ha!”—startling Mr. Clark into missing a crucial pop-up.

But there was no question about what the Post had hollered from the newsstands that morning: STRAY-ROD was the wood, accompanying a “photo exclusive” of Mr. Rodriguez out on the town in Toronto with a woman who was not his wife.

Extramarital activity by athletes is as old as professional sports itself. Under an informal agreement just as old, the press has ignored players’ off-field dalliances, unless the courts get involved or they somehow impinge on playing time.

The Post—or more specifically the Post’s metro desk—decided that the agreement no longer applied to Mr. Rodriguez.

It set off a feeding frenzy.

In the days following “Stray-Rod,” the Post and the Daily News covered the story relentlessly, with front-page stories and back-page spreads on May 31. A concurrent negative plotline focused on allegations that Mr. Rodriguez had engaged in unsportsmanlike conduct on the field.

Even The New York Times was forced into semi-participation, hanging a daily story about Mr. Rodriguez’s tough week on the fact that Mr. Rodriguez was asked “questions about his personal life.”

The effect of the Post’s decision to go, essentially, where its competitors wouldn’t was seismic.

“When I saw it and heard it, it was like, ‘Wow!’” said Buster Olney, a former Yankees beat reporter for The Times who is now a writer at ESPN the Magazine. “This was very, very different from my own experience in covering teams. It felt like the line had been moved.”

Mr. Olney said that when he covered the team, he was aware of a well-known married player who sired a child out of wedlock. It did not make the papers.

So how did the “Stray-Rod” story cross the line? And how did the Post come to achieve its big scoop?

It started when a still-unidentified paparazzo snapped photos of Mr. Rodriguez on May 27 entering a strip club in Toronto with a blond woman who was not his wife.

The photographer offered the pictures to the Post and, according to staffers at the Post’s crosstown rivals, to the News.

The Post decided first to go with them.

Greg Gallo, the Post’s sports editor, said the decision to run the front-page story was made by the Post’s city editors.

“It really is their story, not ours,” he said in an interview on June 4.

He said there was a “back-and-forth” between the sports department and the city editors on the story, and that he was “brought into the loop” on the paper’s decision to run the story on the front page—but ultimately, it was “their call.”

Once the story of Mr. Rodriguez and a “mysterious, busty blonde” broke, other photos quickly emerged of him with the woman, a gym-hardened blond stripper named Joslyn Noel Morse, from over the past several months—at a restaurant in Miami Beach, at a café in Tampa, and clad in bathing suits in Las Vegas. There also appeared photos of Mr. Rodriguez’s apparently cuckolded wife, smiling bravely at the attentions of the staked-out hordes. Also, later, pictures of Mr. Rodriguez out and about with his wife.

A Post spokeswoman, Suzanne Halpin, said, “It was the biggest story in town last week, and we’re proud to have broken it.”

Both Post editor in chief Col Allan and managing editor Jesse Angelo were unavailable for comment for this article. And the Yankees, through spokesman Howard Rubenstein—whose company also represents the Post and has done public-relations work for this paper—declined to comment.

Daily News editor in chief Martin Dunn, who said that he was in England over Memorial Day weekend when the incident took place, offered this statement about the chronology of events: “We don’t comment on what unnamed sources say. But the News was in the process of trying to verify the identity of the woman when the photog asked for his pictures back.”

One of a Kind

What is now abundantly clear is that there is a unique set of rules that apply to coverage of Mr. Rodriguez.

“A-Rod is in a class by himself,” said New York Post columnist Mike Vaccaro. “In the context of time, Patrick Ewing comes closest, but he was certainly not at this level.”

But close attention from the New York press—particularly of the negative variety—is nothing new for Mr. Rodriguez. He was criticized in the past for seeming to express ambivalence about being in New York and, oddly, for getting caught by a photographer sunbathing without a shirt on in Central Park. And earlier this year, converging storylines about Mr. Rodriguez’s alleged cold feud with the talismanic Yankee captain Derek Jeter, about his apparent unhappiness in New York and, bizarrely, about an incident at a children’s-book reading by Mr. Rodriguez at which a reporter was manhandled, all came together in a perfect storm of public disdain.

Put simply, the star was being bullied, and he couldn’t figure out how to make it stop. It seemed as if he wouldn’t even make it until Opening Day, let alone last the season.

There was a respite once the games started and Mr. Rodriguez got off to a blisteringly impressive start.

But then came “Stray-Rod.” The coverage over the last week of Mr. Rodriguez—of his conduct on the field and, more extraordinarily, of his female companion—has been something else entirely.

Even by the standards of the toughest sports-media market in the country, the daily barrages aimed at the Yankees’ handsome, hyper-talented third baseman have been nearly unprecedented in their scope, and almost certainly unequalled in their visceral, almost gleeful negativity. They haven’t so much brought to mind the scrutiny of Joe Namath or Reggie Jackson in their athletic heydays as they have that of O.J. Simpson, post–bloody glove.

Of course, other athletes’ marital problems have provided tabloid fodder this past year, including Giants defensive end Michael Strahan, Nets point guard Jason Kidd and Mets catcher Paul Lo Duca. But each of those scandals stemmed from an already-public divorce case, not from a random paparazzo’s telephoto lens.

So what made Mr. Rodriguez fair game? And why is he such an irresistible target?

There are a number of theories, some less rational than others.

One of them—a complicated one, given the fact that Mr. Rodriguez was voted league M.V.P. two seasons ago—is that he has performed disappointingly since arriving in New York in 2004.

Though he currently leads the American League in home runs and ranks fourth in R.B.I.’s, his spectacularly high contract—$252 million over 10 years—has rankled, as has the fact that his team hasn’t won a pennant since he joined them. Worse, he has acquired a reputation as a choke artist: In the playoffs, he has batted .103, with no home runs or R.B.I.’s since 2005.

Reporters, like the team’s fans, hold him to the standard of Mr. Jeter, a four-time World Series champion. Inevitably, he falls short.

“If he led the Yankees to a World Series, maybe people would leave him alone,” said Murray Chass, the veteran New York Times columnist. “It’s a cumulative effect: When everything doesn’t go perfectly, it’s held against him, and once you have enough of those things, the dam bursts and you get this.”

“If he had been perfect on the field and had not made some of the comments he’s made, it would be hard to create something there,” he added.

And about those comments: Mr. Rodriguez’s attempts to communicate with the press, whether he’s being deliberately minimalist or toying with expansiveness, have had an almost magical enraging quality on the reporters assigned to cover him.

“He hasn’t been really hostile,” said Tom Jolly, The Times’ sports editor. “He’s generally not terribly quotable. He’s certainly not a quote machine. It is a bit of a mystery that we’ve tried to address on several occasions: What it is about him that rubs certain people the wrong way?”

“It’s an equation—a little of this, a little of that—and Alex has managed to accumulate it all,” said Filip Bondy, a columnist for the News. “He has the talent, and he has the controversy, and he has the huge contract, and he has the attitude.”

The idea that Mr. Rodriguez is a cheater—based on the thinnish evidence of about three on-the-field plays, the most recent of which was the “Ha!” incident—somehow completed the toxic mix.

Still, in the wake of “Stray-Rod,” the sports writers’ explanations can only amount to best-guess rationalizations for a saga that has suddenly—literally—become front-page news.

After all, said a reporter who’s covered the team this season, “all the Yankees writers knew about this woman.”

“Because we’re on the road, and keep the same hours, we see plenty of things that we don’t want to write—and we don’t write it,” said Tyler Kepner, the New York Times Yankees beat reporter.

“We don’t go around spying on them,” Mr. Kepner added. “I’d like to think that we never will. When the paparazzi gets involved, then the story takes on a life of its own.”

“The general policy is that we try to limit what we report to whether it affects his on-field play,” explained Mr. Jolly. “So that’s why we normally don’t delve into that sort of thing.”

Mr. Jolly suggested that there were multiple explanations for the special scrutiny.

“There are a number of factors,” he said. “One, he’s been the highest-paid player in baseball, which makes him a target of interest. He’s a handsome guy, a good-looking guy—that makes him attractive to people who don’t follow sports. And he invites it himself by being very public in some of his outings.”

But still, running a story that didn’t occur before the eyes of a reporter—as it presumably didn’t happen in front of Post general-assignment man Dan Mangan, who declined to get into the specifics of the reporting that went into the piece—seemed to puzzle some baseball reporters. Reggie and Billy fought in the dugout; Doc and Darryl got arrested in the public eye.

Then there was Mr. Rodriguez in Toronto, not viewable through court documents or the press box.

“I think if the baseball media perceived any of this as a story, it probably wouldn’t have taken this long,” said Mr. Kepner.

“We’re in an era where I think the landscape has been changing,” said Leon Carter, sports editor for the News, defending the paper’s coverage. “Anything done out in the open, and in public, is fair game.”

No Bobby Bonilla

The great irony in all of this is that Mr. Rodriguez, for all his flaws, hasn’t done nearly as much to earn the press’ hostility as other athletes who have played here.

Former Met Bobby Bonilla, another high-priced import, physically threatened a reporter in the team’s clubhouse during the nightmare 1993 season. Later that same season, Mets pitcher Bret Saberhagen tossed a firecracker under a table near reporters, and another time sprayed some of them with bleach.

By contrast, Mr. Rodriguez has at times been a model of restraint. At other times, he has been somewhat prickly. The press has found both versions maddening.

“He doesn’t get it, and doesn’t quite understand the way the game is played with the media,” said Mr. Bondy. “It’s unfortunate, because he’s a living and walking headline.”

Worse, whenever Mr. Rodriguez has ventured to speak extemporaneously, the results have been a disaster.

In an interview on WFAN during spring training, he was asked whether he’d like to finish the length of his contract with the team. Rather than simply saying yes, he offered the following: “At some point, either New York is going to say, ‘I’ve had enough of this guy, get him the hell out of here’—and we have an option—or New York is going to say, ‘Hey, we won a world championship, we had a big year, you’re a part of it; we want you back.’ I also want to make sure, from the fans [and] management, I’m wanted here.”

The Post referred to him the next day as “a heat-seeking missile of controversy.”

Or take his relationship with Mr. Jeter—the untouchable Mantle to his brooding Maris.

Mr. Rodriguez simply could have declined to comment on his relationship with Mr. Jeter—as his teammate had done all along—but instead decided to hold a press conference at spring training to say their relationship had indeed soured.

“He attracts these things,” said Dave Anderson, the New York Times columnist. “I think it’s accidental, too. I’m sure when he yelled at the infielder, that’s something that happens quite often in baseball—but because it’s him, the Toronto players made a big deal of it. He’s a magnet for stories, for better or worse.”

“I think we are in an era when a lot of attention is being paid to celebrities—the Paris Hiltons of the world,” said the News’ sports editor, Mr. Carter. “Now it’s going to be the A-Rods of the world.”