It has a lot more upvotes than other posts above it, per hour, and it dropped something like 15 spots instantly.

It could be buggy too. The "Ask HN" section is chronically messed up, showing low-karma posts from days/weeks ago when there are active, high-karma posts being pushed a few pages under.

Or it could be some over-eager powerful flagger :)

Yeah it has 127 karma and it was just on the front page, I don't think flagging has that much weight

Depends on who does the flagging, I think. Some flags are, like, an instant 10-spot demote. Some kick it from the 'news' section of the site completely (but still visible in 'newest').

I suspect there's a 'small flag', too, that doesn't drop the post's ranking unless enough small flags are accumulated.

It's all a bit blackboxy :)

Maybe someone flagged it, but probably not. Mondays are just highly-active times - posts rise and fall quickly.

Crazy, I don't even see this post in the ask section anymore.