I'm going to kill this in the meantime. As it says in the guidelines, you should send moderation questions to us, not post them on the site.
1) A couple other users clicked the 'flag' link on the submission. Once you pass the 'too-many-flags' threshold, the post drops position drastically and immediately. Any user with sufficient karma can flag submissions.
2) A moderator flagged the submission. PG is not the only moderator.
3) Some portion of the 24 people that voted for the story were detected as part of a voting ring. The voting ring detector flagged the story.
I would love to know though...
It could be that we had high comments / low points? I really don't know, it just doesn't make much sense.