I don't think the real cause of a SERP shakeup is evident yet. It's fairly strongly suspected that something in the PR algorithm was changed around the time of the last PR update. Many pages in a variety of subject areas lost PR during this update or didn't gain as much as suspected. Many folks feel that the PR "levels" were ratcheted up - others feel that G is weighting links differently. Right now there is a lot of speculation but not a lot of fact (if that's even possible). Some people saw older, more established pages with entrenched link structure gain ground in this recent update. Others saw older pages with an entrenched link structure lose ground. Take a look at this page of free directories:
http://directorylist.sitedesigntutor...ortpr.php?pr=0 - just see how many slipped backwards in PR.
Another factor involved is that, like the real estate market, changes take time to reverberate. Things are not settled - even from the last PR update so any conclusions anyone can draw are going to be hypothetical at best.