Maybe it was the cowardly teammate who anonymously ripped him in print. Maybe it was the absence of a game-breaking wide receiver or running back. Maybe it was the prospect of playing for a fourth different head coach in five years.
Or maybe Brett Favre simply decided it was time. Time to retire after defying age and injuries for 18 seasons in a league where NFL means Not For Long.
This is Favre's second retirement in 12 months. There are those who think he should have stopped at one -- his March 4, 2008, supposed career-ending announcement with the Green Bay Packers. (Remember all that management nonsense about wanting to protect his football legacy? Sure, just as long as that legacy didn't include Favre's playing for, say, the NFC North's Minnesota Vikings.)
But Favre did a reversal, unretired, got traded to the New York Jets, won nine games (three more than the team that traded him), played with a torn biceps tendon, missed the playoffs and 45 days later called it a career.
It wasn't the way he wanted to go out -- an 8-3 Jets start, a 1-4 finish -- but his legacy survived just fine. I think he could have played another year. So did the Jets and, I'm guessing, so did Favre.
And maybe he would have returned had it looked like the Jets were going to sign linebacker Ray Lewis, or make a real run at wide receiver Anquan Boldin. Maybe, but who knows for sure? Plus, it's not like Jets fans, numbed by 40 years without a championship, were clamoring for his return. It was more indifference than anything -- perhaps on both sides.
I'm glad Favre played in 2008. I just wish it had been for the Packers. Still, in a different uniform, with different teammates, coaches, playbook and opponents, Favre finished 9-7. That's five more games than the Jets won a year earlier.
This wasn't Favre's finest season. Not even close. He led the league in interceptions, nine of those 22 INTs coming in the final five games. But to blame Favre for the Jets' collapse would be like blaming Aaron Rodgers for the Packers' collapse. It's convenient, but it isn't completely accurate.
Compared to the Jets' 2007 season, Favre's 2008 completion percentage, touchdowns, passing yards per game and passer rating were all higher. The Jets didn't get a playoff spot, but Favre made the Jets relevant again. Secondary ticket sales spiked. Media attention spiked. His Jets jersey was the NFL's No. 1 seller in 2008.
Favre did what he could. Five years ago he might have been able to do more, but Favre at 39 was still better than a lot of quarterbacks in this league at 28.
Was he a bit of a drama king? Sure. So what? But he deserved better than the Jets weasel who anonymously cheap-shot him in the papers at season's end.
His first NFL completion -- to himself, on a deflected pass -- came on Sept. 13, 1992. The last completion of his career, made Dec. 28, 2008, was equally weird: a 4-yard pass to Leon Washington, who lateraled it back to Favre, who then attempted an illegal forward pass to Jerricho Cotchery.
In between that first and last completion is a career deserving of a ticker tape parade. No NFL quarterback has thrown for more yards or touchdowns than Favre. No quarterback has more wins, completions or 3,000-yard seasons. And no quarterback has played more consecutive games (a staggering 269 regular-season games ... 291, if you include playoffs) or won more league MVPs.
He played with torn biceps, bruised elbows, numbness in his fingers, hand sprains, concussions, bruised hamstrings, a broken thumb on his throwing hand, sprained knee ligaments, elbow tendinitis, ankle sprains, bruised hips, separated shoulders, coughed-up blood and a broken heart (when his dad died). It's a miracle he doesn't have to glue himself together each day.
Favre threw 464 career touchdown passes to 50 different players. If those 50 guys are smart, they saved each of those footballs.
He was a second-round pick in the 1991 NFL draft. Thirty-two players, including two other quarterbacks, were selected before Favre. Favre is going to the Hall of Fame. Dan McGwire and Todd Marinovich, not so much.
One of these days, Favre and the Packers will kiss and make up. It was a messy divorce that could have been handled better by both sides. But in the end Favre is a Packer and always will be a Packer. The clumsiness of his forced departure shouldn't change that.
ESPN's crack research staff compiled a 15-page analysis of Favre's career. One of the pages featured the top 10 Favre games, beginning with his one and only Super Bowl victory. It's a good list, but what I'll remember about Favre isn't so much the games, but the way he played the games. His beard was gray, but his attitude was high school pep rally.
The NFL isn't going to be the same without Favre -- and how many players can you say that about? More than each of his records, that's the Favre legacy that counts.
Gene Wojciechowski is the senior national columnist for ESPN.com. You can contact him at email@example.com.