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.
Could be just regular users as well. Anyone who has enough karma can "flag" a post, and a few flags will typically knock something off the front page (unless it has a ton of upvotes counteracting that).