A lot of sites have SEO issues, but this is not what caused the drop. It has effected a large number of left-wing sites (http://www.wsws.org/en/articles/2017/08/02/pers-a02.html and http://www.wsws.org/en/articles/2017/08/08/goog-a08.html). It also happened directly after Google's new search policy was implemented.