I feel at least a good amount of it came from Channers and their obsession with trolling/harassing/stalking people online (e.g. a certain person who has an entire wiki dedicated to them). It doesn't really matter to those people if any of their targets actually have autism, it just became a catch all term for anyone "different" online that generated "cringe." Then it basically became an insult people would use in arguments used interchangeably with the r-slur.

Edit: to be clear, I don't think autism is "cringe," I'm using the terminology of people who liberally throw around autism as an insult.