I'm going to continue what LadyPianist said:
In the neck, there are some nerves and stuff that control the balance of the whole body.
Now, some people have this habit of moving around a lot. What I think here isn't very important, cause everyone, as I said, think differently of this. As long as he moves from the back, with a straight neck, it's ok. But if he continues curling into a ball, that can be dangerous for real, and his sense of balance (not in music, obviously) might actually be damaged.