If you read any of the forums you’ve probably heard murmurings of PR update underway. Actually this ties in quite nicely with some sandbox phenomenon I’ve been seeing for quite a while. Usually about 2 to 4 weeks before a pagerank update goes public sites start popping out of the sandbox ( see Peek-a-Boo in the Sandbox).
My current working theory goes something like this, every 3 months or so Google recalculates certain ranking values for your site. These would be things like trust rank, quality rank, link count, etc. They then compare these values to those from the last calculation period. If you’ve earned enough trust and quality to get out of the box they compare your site this period to last period and see if you fall within the acceptable range, if you do you’re set free. Now I’m not sure what happens for people who have “excessive growth” but if I were Google those links would be the ones I’d send to the skunk works area 51 people that are part of the non existent human evaluation team.