I'm going to put on my psychology hat (which I clearly stole from some egg head), and say it's related to the bully mentality. Football is not just an extremely physical sport, but one where the culture of how to win is based on the idea of imposing your will on the opponent. Skill, tactics, etc go right out the window if one team is simply bullying the other through tough, physical play.
For that reason, players amp themselves up before a game. They look in the mirror and know that the only way they're going to win is to be tougher, meaner, and WANT it more than the other guys. They yell at themselves to be that guy, they psyche themselves into believing that they have what it takes to destroy their opponent.
Then they get embarrassed 45-7.
Now they have to come down from that high. It's not just humble pie, it's a whole god damn humble buffet (a humbuffet, if you will), and they have two choices: Except it as part of the whole sporting experience, or lose their minds and look for whatever convenient excuse they can find (cheating, unsportsmanlike formations, bad refs, league conspiracies) that can convince them that they WERE tougher, meaner, and wanted it more. They just got screwed by something out of their control.