In hindsight what happened for us was the Mayday Update. While we did update our site, what happened to us specifically was some of our long tail keywords lost ranking. This freaked me out at first, but as I have had a little more time to process the data, I can see that our conversion rates have actually gone up.
Some of the traffic we were getting pre-Mayday was not as targeted, and did not convert well. We've continued tweaking the site, and if anything it has been a great learning experience that led us to improve our site.