I suspect a lot of the casual miners who came in with the last big round of press hits are getting out. Those who were CPU mining for a month or two and made less than they spent on electricity, or the NVIDIA miners who barely broke even, or even lost. At this difficulty such mining isn't really worth it.

And, summer hit with a vengeance here in the US on the day before the difficulty rose. I suspect some of the miners just can't stand the heat anymore.