Its based on the discussion it generates. If it generates heated discussions with little thoughtfulness or curiosity, it gets flagged and removed. Its effectively the opposite of how Facebook / Reddit / similar clickbait platforms work, and one of the key reasons we don't get as much low quality garbage comments here.
cloverich described the theory. In practice, if enough people loosely band together to flag, they can mark an entire story as flagged. As of late, almost every topic related to Elon Musk became flagged, so I wouldn't be shocked if there's a more dedicated brigade for this (as well as a mix of the usual "I don't want politics in Hacker News" crowd).