Probably the posters involved. Blessing and a (minor) curse that HN doesn't use avatars. Harder to avoid commenters you know you don't like.

If the number of comments in a post or subthread exceeds the upvotes, this leads to the flame detector activating.

But also some accounts are just banned.

Banned accounts can still comment, but they start dead and have to be vouched first before showing to anybody who didn't enable showdead in their profile.

I have noticed a trend that some of my friends accounts get rate limited extremely easily while others don't. Not sure what's happening there, I assume there's a flag set by a mod or its otherwise calculated by karma total number of times the account has been flagged. it doesn't seem to correlate with recent controversial posting