alexa changed the way they measured their stats around that time, they dont rely exclusively on the toolbar any longer, a lot of sites were affected
Alexa Changes Rankings, Goes Beyond Dreaded Toolbar
April 17, 2008 ? 12:07 AM PDT ? by Stan Schroeder ?
The folks at Alexa have announced that their ever so popular (but, many might argue, often inaccurate) ranking system for web sites has been altered. The official word on the changes is slightly vague: Alexa claims ?better rankings,? and ?improved methodology,? but doesn?t go into detail.
One change, however, is definitely a big one: Alexa is now using several sources for its rankings. So far, Alexa has measured rankings of web sites according to the users of their Alexa Toolbarhttp://en.wikipedia.org/wiki/Toolbar , and this was the main source of users? and experts? criticism. For one, the Toolbar is not available for all platforms (you can?t get it for a Mac, and that?s why Mac-oriented sites have always ranked poorly on Alexa). Secondly, the Toolbar was reportedly mostly used by the tech-savvy demographic which are interested in rankings, and therefore various technology and SEO sites were perhaps unfairly well ranked
http://mashable.com/2008/04/17/alexa...eaded-toolbar/