One other thing to note, the problem site had a PR3 before the last update. The other 3 sites had PR1 or lower and all jumped up.
i am not an expert in the subject, but as far as jumping of other sites is concerned, pls check February/March posts in this forum. IMHO, the last update was not a complete update, just improvement due to swithcing over to new infrastructure. lots of new pages created between december and march showed steep rise in PR but older pages remained almost same.