Rob Kerry about the pengiun update of Google. He is also saying that anchor text spam could bring down the rankings of a site.
How to get back the rankings after the penguin...
At the end of April 2012, Google has deployed a ranking update called “penguin”. The penguin is eating a lot of fish from our site. The traffic of schicksal.com has decreased by 50% from one day to another. (Before this update we had about 200k visits per month, after the update we would be happy about 100k visits…) We have lost many good rankings at many keywords. According to figures of...
Negative SEO: Myths, Realities, and Precautions - Whiteboard Friday by Rand Fishkin (seomoz)