tedster - 8:41 pm on May 9, 2013 (gmt 0)
If it's not Rank Modification for Spam Detection?
My guess - it's a long a slow process of new data integration in the the rankings. Matt recently made a few comments about that kind of thing to several webmasters. What a tangle it sounds like Google has going, eh? No wonder he suggested we will not be able to reverse engineer that algorithm anymore!
I'm looking for a reference right now - don't buy what I say unless I can prove it.