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I checked the Panda dates, but the only one that looks interesting is July 24th which is the day after the highest point on the graph, but that was only a data refresh.Panda 3.9 affected about 1% of search queries and according to those gurus, it's still significant.
Panda - go back and check your site for any dupe, thin, redundant content. If you de-optimized your site recently and have gone through a significant site design, chances are you also had a full recrawl and made Google evaluate your site in a new light.
Backlinks - You mention you have WMT. You are able to get a feel of some of your backlinks on there. Check that out and see what kind of keywords and sites are linking to you. Others have also mentioned the free tools by the 2 other sites above. Anything there?
--"red widgets for lemmings who love squirrels": #1If your product is indeed appealing to lemmings with strong preference for squirrels, why would you NOT want to rank exactly like this? For an ecom site at least, I think it would benefit everyone (you, visitor, Google) if you get 1 visitor who can actually buy rather than 1000 visitors who are not even interested.
If your product is indeed appealing to lemmings with strong preference for squirrels, why would you NOT want to rank exactly like this?