This seems more of a link algorithm with a mix of keyword rich urls update.
We also track around 20,000 keywords on a weekly basis and can see a big flux. This seems more like Google devaluating some of the backlinks or sites in masses which is causing this.
There is no specific pattern which we can see. It seems more of the small business websites getting hit than the Brands.
But again too early to single out specific reasons.
