It happens at 25th april 2012, but its not clear till now that it is over optimization or web spam update.
this month some site experience decreasing rank and backlinks and it's because of the new algorithm that Google launched.
Yup, because of this also i have found some site's backlinks got decreased suddenly. This information really helped me.
The Google new algorithm is for “black hat webspamâ€."The change will go live for all languages at the same time within the next few days", said engineer Matt Cutts in his 24 April blog post titles-Another step to reward high-quality sites.Cutts said the changes are targeted at sites engaged in tactics such as keyword stuffing, or “unusual linking patterns†where unrelated links are sprinkled throughout a fake or manufactured article.
google new algorithm is a real panic for my website my page ranking gone to 10th page, and this update really sucks.