That there "could" be a severe Spider neuro problem. BUT, I don't think the snake would have thrived and grown that big with that severe of a problem.

On top of that you say that isn't the worse he does, and that he does it even when everything is low key, low stress.

Is he in a rack ? Is it a bit darker normally?

What I would do for now is to give him a smaller waterdish and keep it somewhat shallow. Enough to drink of course, but so he can't drown himself. Keep him in a dark place or give him hides. It may calm him.