I think you're underestimating how arrogant Pete can be. He probably thought the game was in the bag, and he wanted to show out a little with Russ and did something that
@Peter Vecsey called, "uncharacteristic" and it certainly was.
Ask yourself this, if you thought you had the game in the bag, and you kept hearing shyt from the league about keeping Lynch "in line" or whatever, would you want to risk Marshawn on the ceremonial podium in front of millions of people around the world talkin bout "I'm thankful....I'm thankful"?
What other reason would they force that dumb ass play, that a rookie who didn't even play all season, saw coming a mile away? Just answer me that.