My first thought was that the chain broke but if it is intact it was either a defective derailleur pivot pin or something hard ( a piece of wire perhaps) got picked up by the chain and run through the derailleur.
I distroyed a nearly new Ultegra rd a few years ago when I rode over a short piece of coat hanger wire and it got flipped up into the chain. The chain survived but the rd was a twisted wreck.