This is less likely than my scenario, IMHO. The chainline is dead straight, the chain wasn't loose, and the chain and cog are both fairly new and in good condition. Throwing the chain without a lateral external force seems really unlikely. Also, to lend further credence to my theory, when I was walking my bike in the garage, the back wheel suddenly started dragging, because the broken spoke got caught between the chain and cog. Anyway, I have a wheel to repair.