We know that some of the PageRank changes were due to paid link selling, as we reported was happening back in early October in our Official: Selling Paid Links Can Hurt Your PageRank Or Rankings On Google post. However, not all the sites impacted in recent changes were selling links (several strongly deny this), so there may be other reasons related to the drop.
For example, one common theory is that Google might be penalizing for excessive cross-linking between sites. This is all speculation, of course — and it could be that rather than penalizing for cross-linking, Google might simply not be giving as much credit to some links than in the past. That wouldn’t be a real penalty (as happening in the case of paid link selling), but it could still cause a drop that’s perceived as a penalty.
When asked about other reasons, Google didn’t shed any more light:
Google is always working to improve the ways that we generate relevant search results and update our opinions of sites’ reputations across the web. The Google Toolbar shows an indicator of PageRank, which is Google’s opinion of the reputation of a webpage. Values in the Google Toolbar can fluctuate for a number of normal reasons, including changes in how we crawl or index the web, or changes in the link structure of the web itself. In addition, Google may update the visible PageRank indicator in the Google Toolbar to incorporate not only our view on the backlinks to a page or site, but also to incorporate our opinion of the forward links for a site.
Back to what Andy spotted, sites like ProBlogger have seen their toolbar-reported PageRank drop from 6 to 4 and now back again to a PageRank of 6. Why? Darren Rowse of ProBlogger suggests:
I’ve had confirmation now from two sources at Google that this latest round of updates was ALL about the selling of text links. You can see Jeremy Twittered the same thing here. Some of those who have had their updates reversed today were falsely penalized for selling links (like ProBlogger which used to do it but stopped months back).
Andy is tracking sites hit by this PageRank update and is maintaining a historical toolbar PageRank table at his blog.