I would bet it was some kind of internal conflict. If that is the case, and i want to stress I have no evidence it is, it's totally on Donovan.
Too many of the great teams use players well over 31 (his age) and unless he has uniquely aged prematurely, I can't believe he isn't in good soccer shape.
Many believe he is the best player to ever come out of North America. I agree. I'm sure he has lost a fraction of a step from 27 years old in 2010, but his wealth of experience, superior technical skill make him a mandatory member of the team.
Unless he refused to buy in to something Klinsmann had going on.
That is the only thing that makes sense to me.