I think it could be down to your link profile which appears to be very anchor text heavy when I ran your site through opensiteexplorer.org. Since Penguin has come about link profiles are under more scrutiny than ever and heavy use of anchor text is getting picked out as manipulation for rankings purpose (see - http://searchenginewatch.com/article/2172839/Google-Penguin-Update-Impact-of-Anchor-Text-Diversity-Link-Relevancy.
On the 21st Nov there was another update (Panda #22) to Google that affected a small percentage of websites - http://www.seroundtable.com/category/google-updates.
Have you had any warnings through Google Webmaster Tools?
With Googles Algorithm ever changing I think you would find this very recent whiteboard friday helpful - http://www.seomoz.org/blog/prediction-anchor-text-is-dying-and-will-be-replaced-by-cocitation-whiteboard-friday
