Google PageRank Drop
A drop in PageRank is usually due to the site violating 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 the above linking tactics as a clear violation of its quality guidelines, and is the main reason why PageRank drops which will also result in a decline in website trust.