No one here is a Google executive to explain that! It is all Google trickes and mood, so lets not think about it.
I personally think it has to do with losing pr links to your site. Every pr valued targeted link counts to building your pr and allowing you to get in the organic search result of google.
There might be also an algorithm change . I am pretty sure that with the same linking and setup , a site would rank differently 5 years ago . Also , from what I read, the PR reuirement progresses as the web grows , because more pages are created .
The PR of some of your links decreased or was lost completely therefore your link on their site(s) has lost weight. Your previous PR was probably borderline if this happens imo so you really shouldn't require many more links to regain that lost PR on the next update. Wouldn't worry so much, the next update you should definately have your old PR back if not a better one. .
It is usually caused by factors disvalued such as Recip Linking however PR still goes by quantity not quality. Quality has a very little piece of PR.