Did you build new links? If so, what kind of links?
I read somewhere that to have a natural link profile, a site should have no more than 30% exact match anchor texts, around 10% partial match anchor text....and 60% of generic anchor text/naked URLs.
My site was hit, and I have like 60% of backlinks with exact+partial match. (Considering sitewide links as one)
I wonder what role does PR plays in all this.