I don't have any direct input or evidence, but I have a guess. Could it be related to the rise in bots? I see many posts which, to me, seem rather low quality (not that I'm the final arbiter of quality or anything) but which have rather high payouts. Often, these are posts which bots have upvoted. I don't know much at all about how the system works, but it seems as though this would damage the credibility of the upvote system and lower overall payouts.