With each update Google finds more pages and with each increase, the “average” number of links required in order to maintain any given PR level grows. So even without losing links, if you don’t continue to “grow” your backlinks at the same (proportional) speed as the web grows, your PR will gradually dwindle away.
http://forums.seochat.com/google-optimization-7/pr-update-shift-down-to-twitter-feed-293167.html