>>HPMOR+LN
As I understand it, HN has a controversial submission detection system of sorts. The exact details elude me, but if a submission gets a lot of comments quickly relative to upvotes, it'll fall off the main page.
Best indicator I've seen is if comments/points ~> 0.9 or so.
It's my understanding mods can undo this "controversial" flag, so that select threads get back onto the main page.