I'd check all that was said above. Perhaps your chainring bolts are loose? That happened to me once.

I'd also check your chainstays. Could be cracked. Wasn't this similar to what Lance had a couple of tours ago when he was mashing the pedals? But then again, he only slipped in his pedals and not a thrown chain.

Or maybe you're just too damn strong and the mashing on your pedals is causing your frame to flex so much that you are throwing your chain. Or maybe not.