The same thing was happening to me a lot last year in the later part of the year. Each time I dropped the chair it was on a downhill while shifting, I had it checked out at my LBS and they could not find anything wrong with it, not the derauiler, the chain itself or anything. Most of the time I was able to get it back on but once it got so fouled up that I had to walk the bike back to my car when I was out about 2 miles into the trail. I was not a happy camper.

At the end of the season last year I had a complete tune up and had one of the wheels trued up. So far I've dropped my chain once this year. I don't know what to say. I think it might be me. I've been more consience of shifting on downhills and have not dropped it again yet. Knock on wood, now I'll probably drop it tomorrow because I'm planning a longish ride