Here's a working theory... Perhaps Google has rolled out a new algorithm for filtering 'invalid activity'. In this case what I am suggesting is that perhaps they are trialling a new algorithm that is more aggressive at filtering clicks where say... the user clicks through on an ad and immediately clicks back again. The only reason I say this is because it seems in our case the biggest drop in RPM appears to be on ad units that are more likely for a user to click through in this way - prominent ad units, ad units that are blended within content, ad units that sometimes get those kind of ads that are 'tricksterish', leading the user to click accidentally. Ask yourselves if your ad units are universally affected, or if it's just some of them, and if the ad units affected fit this profile. And report back!