Some combination of: 1) Panda, which redirected hundreds of millions in revenue from some content farms to other parties (including, likely, the ether) which do not show up on Google's books 2) Google is being increasingly aggressive about monetizing the Google SERPS. More ads in more places with more prominent billing. Other sites on the Internet have not been aggressively up-monetizing principally because they have…
What is Panda?
Rather than being an algorithm change, it seems to be a process that Google runs only occasionally, which labels entire sites (rather than individual pages) as low quality. Sites with that label are penalized in search results.
It affected many sites, including those that aren't content farms (mine included,) in what many webmasters believe is collateral damage.