15 points in 30 minutes? I have enough experience with HN to be near 100% confident that it should have been on at least the first 10 pages. It was completely missing. Honestly, I'd be nearly 100% confident that with a score like that in 30 minutes it should have been near the top of the first page (like it was one second, and then completely gone the next).

I'd have guessed that it was a part of some sort of filter - perhaps a "voting ring" was detected or something, but I'd really like to hear from a moderator, or something, because this seems a bit strange.