I agree full site audits are essential however it's often by looking at the backlink profile that you can get an idea of the cause.
I've run analysis on 11 sites affected on the 17th and all but 1 have found common cause (and all but two I can say are unique in their field so not a poor quality content issue - the others were ecommerce as often a lot of content is the same between sites due to shared product descriptions)
I had a look in this case and, guessing which pages would be affected, one page had 48% keyword anchor text and the other page 94% anchor text one keyword (when taking only do follow links in to account). This to me looks like the most obvious culprit but I agree you can only know by spending a decent amount of time looking at every aspect.
The reason why I offer a preliminary audit is the more sites I see the more I can find common causality.