A quick hypothesis. For every comment, assume one reader downvotes if he disagrees in every 1000, and every reader can read 20 random comments per story. There

A quick hypothesis. For every comment, assume one reader downvotes if he disagrees in every 1000, and every reader can read 20 random comments per story. There are more people around with 200+ karma than a year ago. If increase in number of comments do not match the increase in people with 200+ karma, there will be more downvotes. And, we don't read comments random. They're displayed top to bottom with already upvoted ones on top, If people usually try to avoid disagreeing with the majority, mid to bottom comments (usually latecomer comments) will receive more and more downvotes.