Friday night I got my Pinarello all ready to ride. This just involved swapping in the rear wheel and pedals and hooking up the front brake finally. But I didn't feel like hooking up the front brake because I'd misplaced the cable so I decided to run brakeless. I was just going to run a quick 15 mile loop that only has two stop signs and two stoplights on the whole bloody thing, so not much stopping anyway. I'd never ridden truly brakeless before but this would be no problems. And it was, until I got to the second stop sign, I slowed down way early, got to about 10 mph and unclipped one foot (still can't unclip great so I do that far in advance) and was going to continue resisting with one foot clipped in and the other resting on the peal. The problem was that once I got my foot unclipped the chain jumped. Not a huge problem, just drag the one foot and come to a stop. Put the chain back on and I'm on my way. From there on out I did all my stopping with both feet clipped in and didn't unclip until I was at about 2 mph because for some reason my brain thought that it was the having one foot unclipped that caused some sort of unequal forces that caused the chain to jump. Now I'm thinking that sounds silly. The chainline is good, but not perfect, not even close to what I'd think would cause the chian to jump, and the chian tension was good so I still don't know what did it.
So it happened to me, it was spooky but not dangerous. Be careful out there.

sounds like murphys law in action.

Is it possible you caught a pant leg? Seems like that could be a culprit if you unclipped the right one first... The last time I threw a chain that was what did it, and the chainline/tension was perfect.

Your brain may have been right. There is always a bit of flex in the crank/bb shell. You don't really notice it except in situations when really mashing, like to accelerate a big gear or when stopping a fixed gear. On a geared bike, if you are in a gear that puts the chain close to the front der cage you can year the chain brush the cage as you come down on he crank. It is entirely possible that there was just enough flex and perhaps the tiniest bit of slack in the chain. There is a LOT of force applied to the crank when you are stopping a fixed gear. I have dropped the chain twice on my fixie, once in a similar situation, hard panic stop. The other time I just apparently hadn't tightened the nuts quite enough, and when I began stopping the wheel shifted just enough to throw the chain.