Yesterday, I replaced my cassette and chain, having ridden the previous ones through the winter, and re-indexed my rear derailleur and checked the limit screws. Today, I had ridden slightly less than a kilometer on that when I noticed the pedaling got stiff in a manner slightly unlike being in a too-high gear, so I shifted to a smaller gear (i.e. larger sprocket), rode about ten meters and heard a snap and a ping.
The snap had been the rear derailleur breaking and I suspect the ping was from the spring in the bit left attached swinging to the extreme position. The derailleur was pointing diagonally upwards, and the chain was on a sprocket near the middle of the cassette, intact.
The derailleur was a Campagnolo Centaur 11, the cassette was Shimano Ultegra CS-R8000, the same size as the previous one, and the chain a Shimano Dura-Ace CN-HG901-11. The brifters are also Campagnolo Centaur 11-speed.
How did my rear mech fail, and what should I pay attention to when installing and maintaining the replacement so as not to have this happen again?
Edit: I received the replacement mech and when I was installing it, I noticed the hanger is bent inwards. I'm not sure if this is a cause or effect.