We've had this problem with the D for several years now. Actually, even though Cowher has this amazing stat about him always winning when he is ahead in the 2nd half, there were signs of it then, to some degree. Here is my theory.
The reason this team's D folds in the later part of the game is because teams have to just throw and throw. And, that is how you beat this D. When teams try to use downs rushing the ball, they mostly fail, leaving them with less downs of throwing the ball. Once teams just use most downs to pass, they get into a rhythm. When going a more traditional attack, they don't get into the rhythm and waste a lot of downs on rushes that don't usually work well vs. this D (or they didn't used to, we will see, McFadden jacked them up.)
Do you remember the 2001 season? (I think). Our first two opponents had potent passing games and BOTH didn't bother with the rush, pretty much all game (The Tom Brady Patriots and then the Rich Gannon Raiders). Both teams ripped the D to shreds and we couldn't stop them all game. I believe that teams behind in the 4th go into the same mode, of just passing it all over the field, and it works, as that is the way you beat our D. GB did it to us in the Super Bowl as well. As games enter that phase, QBs get rolling and our pass rush gets gassed and that's a bad combination. And, let's be fair, all this chat about how horrible Palmer is, well, that's not entirely true. He is a pro bowl QB and this is his first full year with his new team. Yes, at one point, he was overrated, but he isn't chopped liver either. He has a nice, strong arm and led the AFC in passing heading into this game. Plus, he is very familiar with Pgh's D.
Any how, that's my theory one why this D often crumbles down the stretch.
It's not like the players just don't care and quit trying. They want to win more than we want them to win.