I mean there doesn't even need to be a reason for companions to become illithid.
Make the ones, who slurped tadpoles the whole game the illithid, because they can't resist. In my game that is Astarion or Minthara, since they don't need convincing. So if you are lucky and have your slurpers with you, just let them get turned by default, including your main character. I think the tadpoles should have more consequences.
I know, this won't be happening anymore, but I wish, this would be a thing. You probably have a nasty surprise in your first playthrough, but after that, you have basically control over who turns. And first playthroughs are rarely perfect anyway.
