I made a graph of deleted posts on the site per day (day of post creation, not its day of deletion):
The spike on 2012-06-28 is because of Answerama, a Stack Exchange promotion that did not turn out well. The left edge of the graph shows when the site first launched. It makes sense that many of the first posts have been deleted, since the site scope wasn't well defined at that time.
But what about the spike on 2015-08-22? The list of deleted posts isn't very enlightening: it's mostly multiple answers to a handful of unrelated questions. For the deleted questions where I can see the title, there's no obvious theme there either.
My guess is that many of these posts were due to a very busy troll, but maybe the site had a flurry of activity that day for some other reason. Is there an explanation for this that mere mortals like me are allowed to know?
(In case you feel like that's a lot of deleted posts, Stack Overflow has been known to delete 5000 posts per day. Hopefully, many of those deletions were by the roomba.)