About 3 weeks ago, google had a major shift in the algo, it appears to be with links, being devalued.
The problem is that imagine a top site gets put on the 'does not pass PR' list (it happens all the time), that has an affect across the web, as the loss of PR propagates across the link web from that site, all the sites losing PR and the associated link juice.
OK now imagine that google,having seen positive results in their test, decide that rather than fight hand to hand, they will introduce this change into their main algorithm. BOOM. Pr across the web is altered, propagating from site to site causing that site to bounce around in the serps like billy ho, until, finally when the wave has propagated across the web, it settles.
Looks like that is what happened