1. No real kill switch (jack-of-all-trades).
2. Injuries.
3. Fighters catching up.
4. The lingering effects of an ego implosion from being soundly beaten at his peak, and then getting fired.
All of this leads to a lack of hunger in a game where there are fighters starving for a spot; especially in his division.
In other words he has Rashad Evans syndrome.
You have to be motivated to to get through a brutal training camp, cut weight, and nurse lingering injuries. You then have to convince yourself that fighting someone you don't even hate somehow makes sense.
It's a heavy burden. The fighting part really is the easiest part.