Unlike other updates which attempted to remove "web spam" from the search results by tweaking some of the parameters, this update (according to many in the SEO community) is an active attempt to catch people doing black hat or over SEO optimization. Unfortunately many legit sites have gotten caught in its net.
You can read about it in the WSJ among other places:
http://online.wsj.com/article/SB1000142405270230350550457740...