If I had to guess, the algorithm puts too much importance into upvotes and not enough importance into newness. It is relatively rare to get more than 8 points in any particular sub, but if you get more than 10-12, you'll probably be on the front page, which boosts post visibility, allowing you to get into the high teens or low twenties, causing your post to be visible for too long, as the algorithm thinks it it still popular. There's only one obvious solution, and that's for us all to commit suicide.