I really strongly had the impression that it was a combination of flags, some sort of metric of polarity (lots of upvotes/downvotes among the comments, and then

I really strongly had the impression that it was a combination of flags, some sort of metric of polarity (lots of upvotes/downvotes among the comments, and then moderator actions. I don't recall why I have that recollection, but apologies for any mistaken impression. It's probably better that it's not entirely transparent, either.