Google is constantly adjusting their algorithm, and those changes affect many, many SERPs. In fact, this summer seems to be a bit more volatile than, say, back-to-school or end-of-year holiday time.
So what I'm saying is something like "don't take it personally". It's not like some Google employee looks at a specific search result and then decides to move a particular URL up or down. And given the complexity of Google's ranking formulas, it's not likely that we can reverse engineer an explanation for any one change.
One clear goal of algo changes is the ongoing battle against webspam. And sometimes a site just accidentally has a characteristic that is in the neighborhood of a current spam technique -- whoops, we have collateral damage.