|I don't know for sure, but I'm guessing that it may have a lot to do with Hummingbird. |
I'm thinking that because of the non-converting foreign traffic, which I'm seeing widely reported.
Such traffic usually accompanies changes or tests in the way Google is looking at user intent... and the big new thing right now, which is to say Hummingbird, is all about new ways of looking at user intent.
During such tests, ranking changes are generally not very obvious.