Hey, A-Rod! Smile!

On May 30, Yankees third baseman Alex Rodriguez and the New York Post each broke the unwritten rules of baseball.

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?

Hey, A-Rod! Smile!