Perhaps it just was sent out a little early for some unknown reason as April 27 was also a Thursday.
As for trying to figure out what has been happening.... My uneducated theory is that it has something to do with backlinks.
While most every phrase I used to rank well for took a serious tumble, I can see a trend for which terms took a more serious tumble. The terms that dropped about 150-200 places in the serps (from the top 10) are terms I didn't have many backlinks deep linking to those pages.
While phrases that dropped 400 or more places in the serps (again, from the top 10) were to pages that I did get several backlinks from. Going through about 50 or more phrases I can see an almost 100% correlation showing the terms I dropped further for were the ones I had more backlinks from. Perhaps these backlinks looked unnatural to Google.
What is interesting about this update is that it has impacted sites that had zero content changes over the past several months. This leads me to believe that whatever change is happening isn't related to on-page factors.
Something else I've noticed is that several sites I've seen that were impacted have a 301 redirect in place from www to non-www or vice versa, AND there were some issues with the 301. For example, I had a 301 problem with my site and didn't know about it for several months. There was a conflict with another rewrite rule that was causing lots of pages to get redirected not to the www version of the same page, but to the homepage instead.
So I had hundreds of redirects to the homepage from other unique pages. I can see how this would look weird to Google and I fixed the problem. This was in late July however my site didn't come back on Aug 17.
When I think about what MC has said about data refreshes it seems to be some sort of calculations/evaluations of off-page factors. From a data management point of view it seems to be something that is too complex to be calculated "on the fly". So instead it is calculated every month or so and pushed out.
So assuming this isn't actually a bug that is impacting these sites but rather a deliberate change, I would assume we are looking at off page factors being updated.
I would be curious to hear if other people experienced the same type of drops in the serps.... that the pages you had more backlinks for (or perhaps the pages you had more backlinks with the same anchor text for) the further the pages dropped in the last data refresh.