I'm also very curious about it. There's obvious damage to several links, but without seeing all the parts there's no way to say why. Even with the whole motor in my hand I don't know if there would be a way to know why it happened. Your guess of something being dropped in while doing the work makes sense and it was my first thought, but living for another 500 tack miles after that doesn't quite jive. Maybe the chain was levered onto the sprockets and some initial damage was done that then damaged the guides. It's also possible that when the rod went the shock load snapped that link, but there appears to be too much damage for the link to be an after-effect unless he continued to run it to get off the track. It's also possible that everything was done perfectly and the chain just broke for no good reason.