Overheated discussions get demoted. I think the idea is that the comments should support discussion of the content, but not usurp it.
> The basic algorithm divides points by a power of the time since a story was submitted. Comments in threads are ranked the same way.
> Other factors affecting rank include user flags, anti-abuse software, software which demotes overheated discussions, account or site weighting, and moderator action.
It could also be moderator action.
My most viral submission suddenly dropped from the top story to page 8, despite having far more points than anything else on that page, and only being a few hours old. I suspect this happened because it was a negative post about Amazon. The comments were not overheated. Most posters agreed with my sentiment.
https://news.ycombinator.com/item?id=41821336
https://news.ycombinator.com/item?id=41821400
And that's just on this development. Each stage of this crazy story has had plenty of views and discussion here.
There's flagging a post, but that would show up next to the post - do you have any examples?
https://news.social-protocols.org/stats?id=41791369
https://news.social-protocols.org/stats?id=41815614
https://news.social-protocols.org/stats?id=41821336
Note the orange line indicating rank, which in every case shows a very sudden and precipitous drop in the rank of each post.