Sgt_Kickaxe - 1:19 am on May 3, 2010 (gmt 0)
What happened: First the site lost one PR point from 6 to 5 in the last update. Traffic went up, so I did not care at all, but maybe it completes the picture.
Tuesday, April 27th: solid and good traffic, but slightly lower than last week.
Wednesday, April 28th: first noticeable drop of around 10%
Thursday: 25% loss of traffic to Tuesday
Yesterday: 27% loss of traffic to Tuesday
Think this is the current status: Lost around 25% of power/traffic somehow.
This makes complete sense. Google continually revisits pages on your site so when the index PR went down so did it's pr passing ability to your internal pages. When those were later crawled and your link structure was updated within google those also fell a little. Ultimately some pages were close in PR to a competitors site and your drop became their gain in serps. It's all connected together and it affects everything in ripples, not all at once.
Since you can control how much PR is on your index page (example: all pages link to all pages = flat link structure = all pages rank the same vs all pages link to index but not each other = high ranking index and category pages etc) you can either get more links or offload some category level pr onto the index and article pages. Do this by interlinking articles together a little more, more related items etc. Whatever you change will take a long time to propagate on a site that large but it works.