Regardless of what Google officially says, there is/ was an update around 17 Jan 2013. We track 50,000+ keywords on weekly basis and we saw 6x times SERP movement than we see every week.
For us, this was bigger than Penguin or Panda. Would take some time for rankings to stabilise after which there can be some consensus as to what happened.
Marie - thanks for sharing the link. I kind of agree with you that google will use the disavow tool data. But knowing google, I don't think they will do manual checking for more than 1000 sites? They can simply calculate top 1000 domains disavowed across all disavow requests and then use that data ? But I think that update will come in future. This looks like a version or penguin ?
