This guy just fell apart, physically. He's not the character concern that he was painted as, at times, so it really sucks that this is how it all ended.
He just isn't a two gap defender. Even when he was a pure nose tackle in the 3-4, he was a one gap defender. When you don't surround him with big bodied linemen that can't occupy blockers, it takes away his biggest advantage, which is his speed and quickness to get to the quarterback.