Alright, we've figured out what's going on. The short editing time is an anti-spammer measure; what a lot of the recent spammers are doing are leaving legitimate-looking posts (thus escaping our various spam filters), and then editing the posts to include spam links. (Our spam filters don't seem to catch content after it's been edited.)
Spammer activity seems to have abated significantly after shortening the edit time, though we don't know if it's direct cause-and-effect. So we are keeping the short edit time in place for now so that we can track results over time.