Why has my Google PageRank dropped
Google PageRank Drop
I'm sure you've all heard the above question before.
A drop in PageRank is usually because the site violates Googles Quality Guidelines by selling links that pass PageRank.
Read all about link schemes.
Google take paid links that pass PageRank very seriously especially links labelled as sponsored that pass PageRank. Selling links on multiple pages on a website based on their PageRank and then calling them "sponsored links". Or pages labelled “PARTNER LINKS” which links to the link buyer.
Google class that as a clear violation of its quality guidelines, and is the reason for websites PageRank as well as Googles trust in the website declines.
View the original article by Matt Cutts on PageRank drop.