It's not a coincidence that both things happened at exactly the same time. What are the alternatives, for the same event to cause both a chain derailing and a broken spoke?

The chain was so loose that when he hit a pothole really hard and broke the spoke, the shock also knocked the chain off. Or - Some very solid object, a bolt or something like that, flew into the chain in front of the cog somehow, bound against the spoke and the leverage was enough to both break the spoke and wedge the chain off. I tried that with a pencil and it's possible, it does bind on a spoke. But these two are so unlikely that his idea that the broken spoke derailed the chain is probably the answer.