It's because number of comments > number of upvotes, which triggers flamewar detector.

> How are stories ranked? Other factors affecting rank include [...] software which demotes overheated discussions, [...]. [1]

[1] https://news.ycombinator.com/newsfaq.html

Laat I think I knew anything it took surprisingly few flags and I think people abuse it all the time to get rid of things they personally don't like. And don't like is a broad category.