When teams play lousy, consistently, in the second half, as opposed to the first half, there are generally two reasons.
Either the coaches are not substituting freely, keeping the players fresh, or they are sitting on a lead and are playing conservatively. The best way to lose a ball game is to play "not to lose" instead of playing to win.
It is the continious pressure by and offense or defense that causes the opposing force to break. When you ease up, it gives the other force the opportunity and will to play harder. As in war, it is best to keep the opponent tense, frightened, and ready to give up. Never, give the enemy the opportunity to think they can win.