When good teams come out flat

I've never understood how this happens, especially in big games.

A classic example is the Broncos/Seahawks Super Bowl where the Broncos were 15-3 with all their losses being one score games then snapped the ball over Peyton's head on the first play of the Super Bowl for a safety and it was all downhill from there.

Happened to me in a high school playoff game. We had a good week of practice but in the game nothing went right and we got demolished to end the season.

Seems weird for it to happen in a sport with so many players on a team because it requires a lot of players to not show up but it happens in Aussie rules too.