I think it's time. Hunt has taken much punishment right on the jaw with no probs before. Manhoef - stud though he is - is smaller than everyone Hunt has fought before.
Also note he outwrestled Fedor, and slipped out of an Fed-armbar, but later in his career just let Overeem get him with an easy headlock.
Many fighters have lost their ability to take a punch with time. W.Silva springs to mind. I think Chuck has a bit too. CC himself is far more hurtable than he used to be, etc
I don't imagine it's mental - just the brain itself can only take so much before it starts getting more and more easily ko'd