Thanks for the constructive responses and thanks Shants for that ****ing crazy story. I was feeling sick to my stomach just imagining that.

Many people have said that my chain was too slack... I really don't think that it was! I have seen far, far slacker chains in real life and in pictures on this forum. I will check this **** out today. What about the "once you go slack you never go back" thing?

Anyway I attribute this incident mostly to poor skidding technique. I am also a big guy, 6'3", 215 lbs + loaded bag, and I ride hard. I have a fuji track frame. Could it have flexed enough to do this? How could my chainline be off (stock BB + sugino 75 + surly track hub)? The chainring looks fine, no side to side deformation. I will take a closer look today.