I can't be 100% sure what happened, but what I know is that I heard a "TUNG!" sound, and then my chain was off. The spoke broke at the hub, and there's a bend in the spoke about 1/2" from the break point, and the bend happens to line up quite nicely with where the chain meets the cog on the bottom side. Based on the observable facts, I surmised that the spoke somehow pried the chain off. If there's another explanation, I'm happy to hear it. Otherwise, I want to see the video of Scrod eating his own head .