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.