Google updated pr at the end of october so it could be this. Also as has been said it seems pr doesn't mean an awfull lot anymore. In an another post I have stated in the last 2 months my sites have moved onto the 1st page of google for my keywords yet one site went from pr3 to pr2. Go figure?
It also doesn't update every few months. It updates every single second of every single day. It's an algorithm... An active, living, evolving server based formula the acts on data and time. The only reason people think it updates is because every so often the PR goes public. And that's a fact.
People talking about a "PR update" and people talking about "Web 2.0" are my 2 personal anger inducing pet hates in the online world. (Neither actually exist.)