Something that could indicate trying to dodge, or consciousness leaving the body. It's not unreasonable for Voldie to think "I've lost here, no matter what I do this body will be unusable in the near future, in case he has a plan to incapacitate me without triggering my Horcrux wards I'd better go someplace else".

It is implied that while Voldemort can stop possessing a victim at will, he cannot stop inhabiting his own body at will.

[EDIT]Though Harry internally states the opposite:

He cannot be imprisoned, for he can abandon his body at any time.

This is the part I was thinking of:

The soul he'd created for himself had to be anchored in this brain, it mustn't be allowed to float free.