It reminds me of watching a particularly nasty break-up play out, with emotions running high and lots of bile being spewed.
I dunno - I never especially liked Reddit in the first place, so it's just a thing I see others do. I moved there sort of grudgingly about ten years ago because there just weren't any other visble alternatives, and I've been more or less actively looking for a replacemnet all along, so when the threadiverse took off, I was ready.
But I suspect that for a lot of people, it's essentially that they invested a lot into the relationship, and then suddenly their partner betrayed them, and now they're pissed.