Anyone want to guess what happened here? :-)
What surprised me is that it wasn't already in the smallest cog, but the derailleur moves ahead of the chain so I guess it went into the spokes and never completed the gear change.
Also the way the cage is bent seems odd, but presumably was caused by the tension from the chain once the top jockey wheel had been sucked into the wheel.
(not my bike)
Update: so I'm almost certain it was caught in the wheel. It happened on an open road, no sticks or badgers, and the derailleur ended twisted above and behind the hub, the rider pulled it back down before bringing it in. No broken spokes but some are scratched.