TheOptimizationIdiot - 2:10 am on Mar 9, 2013 (gmt 0)
Any of the 500+ updates a year could cause the type of drop you're seeing without it being Panda.
If you have a page in the top 10 for a competitive one word query, the likelihood of it being Panda is slim in my experience. The sites I've seen get hit by Panda tank.
The difference in "overall score" given to your page in the top 10 dropping from 1 to 10 relative to the other pages in the index (they stop indexing at 1000 pages) is 0.09%.
That's hardly a huge glaring out-of-control drop by any stretch of the imagination.
The rest of your traffic could easily be the same type of drop, like going from an average position 4 to an average position of 11 or even an average of 8 with more results in position 11 and 12 that used to be 9 or 10. Even a tiny percentage of "score" awarded to your pages relative to the rest of the pages in the index can cause the type of drop your seeing.
There's probably some "fine adjusting" somewhere that could get you back where you were, in my opinion.
It's even possible the drop is due to some link network you're unrelated to being totally discounted and the "ripple effect" of PageRank passed to your site from other sites has decreased from it so you don't quite have the ranking power via links you once did. (That's the type of "small" I'm talking about I think it could easily be.)
The main point I'm trying to make is you're not seeing a huge drop going from 1 to 10 in any way.
There's something "relative small" that changed somewhere and to get back where you were in my opinion you're going to have to find a way to "make some small score improvement" on your site, but I wouldn't say you've been hit by Panda, because in my experience sites that get hit by Panda Drop Like a Rock to something like page 5 or 50 or out of the index, but not from position 1 to position 10 or position 4 to position 11 or anything small like that.