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Sorry to say that without an in-depth look, there isn't going to be a way to tell. Unless you changed something, it is most likely testing or an update from Google. I've seen rankings go up and down over the years on multiple sites for a variety of reasons and sometimes what seems like no reason. It can be the negation of the value of backlinks, or an entrant to the market, or Google trying other sites out to see if they perform better in the eyes of the searcher. In this industry, there is only so much you can control. Focus on creating top quality content, focus on the searcher, and you'll come out on top.