I understand he had a poor week of practice, so PART of it was no doubt cloaked in "football-related," BUT: He got in a fight w one of the D coaches before the game. That's why he was benched.
EDIT: I remain convinced that if BB went to him at halftime and said, "GET THE HELL OUT THERE AND HELP," Butler would've gone out onto the field and played like his hair was on fire, and the Patriots would've won. That's my guess, anyway. It's ridiculous that he didn't play at all, especially as the situation grew more dire by the drive.