It had become, over the course of years, ground zero for groups trying to launch their propaganda into the main stream. You see right-wingers and holocaust deniers, homophobes, tankies, russian trolls etc. all trying to launch their social engineering campaigns via reddit.
It's an environment that offers anonymity and unlimited alts, they're basically begging to be sock puppet central. Being in those kinds of idiot pressure cooker communities rots the brain, so it's not a great wonder that it becomes toxic.