It's a phenomenon dubbed the "Brain Eater" by author James Nicoll on Usenet[0]. I don't know what it is about science fiction and fantasy authors in particular - maybe it happens in other fiction genres too and I'm just not aware because I don't read them as much, but a lot of them seem to succumb to extremist (usually right-wing authoritarian) politics, fringe science, conspiracy theory or other such crackpottery at some point in their careers, with their writing sometimes suffering as a result of these beliefs seeping in and taking over.
[0]https://tvtropes.org/pmwiki/pmwiki.php/Main/FilibusterFreefa...