Something happened to site traffic stats (SlimStat plugin) on August 1st: big drop. What is likely cause?

Two web traffic graphs, first is daily for some of July-August, second is the last three months overall. Both have dotted line graphs of the previous period overlaid.

I happened to notice my web traffic stats dropped significantly right on August 1st, and have remained relatively low. My average was 373 views daily, April through July, but in August-September it's been 104.

The potential reason, I'm not sure of, but I'm guessing it is some policy shift that came into effect on August 1st. But from where?

Would it have anything to do with my web host? Or maybe there are search bots that stopped crawling my pages? Or maybe the 'SlimStat' plugin changed the way it measures things? I don't know…

Any ideas appreciated, especially if you know what you're talking about. Thanks!