[jhghikvhu]

Well there are other circumstances too. He didn't defend his classical chess champion title. He likes his alcohol.

He hasn't gone off the deep end (past tense). But is it a process currently in progress? Only possible to say in hindsight but does certainly seem possible.

[devit]

Probably just burned out from chess, especially training the whole day for competitions.

We are animals and eventually the brain will rebel against extreme repetitive mental effort that is perceived to be at least partially useless (and given he's already been the world champion, it's easy for part of him to think there's no point in training).