After a bit more peeking around various keyword search results that I'm familiar with, I am wondering if trademark/brand usage has something to do with this update. Was your website tied into a particular brand/trademark and has been part of this update in a bad way?
I'm not going to be surprised to hear Google come out with something about trademark usage and copyright usage. Something along the lines of the rightful owners getting the traffic as it's rightfully theirs. If you're trying to ride on the coat tails of a brand, did you experience a tumble with this update?