Well, drugs is obvious, but covid is also known to cause cognitive issues: https://theconversation.com/long-covid-puzzle-pieces-are-falling-into-place-the-picture-is-unsettling-233759

I mean there were some signs earlier (of course those who know him will say he always was an asshole, but at least he was better at masking it) it was 2020 when he went full bonkers.