During the first two or three months of 2011, Google cleaned the house with a new update called Panda, which affected 1000's of Article Marketers that used to distribute their articles to sites like EzineArticles.com, HubPages.com and others (like articledashboard, goarticles etc).
This update has affected those sites + every site that google considered as "content farm" (basically, sites with lot of duplicate content,or rewritten content). Yes, Google Now Detects Rewritten Content.
The Latest Google Update ("Penguin Update" 2012) had also a similar effect among 1000's of sites but has more to do with other factors like bounce rate etc, quality of
backlinks to your site etc.