Why did my rear derailleur fail in the middle sprocket? 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.
From the information available I would advise you to make sure:


Chain is of correct length
Derailleur is not running too close to the cassette (B-screw adjustment)


In the workstand pay close attention to what happens with chain on the largest chainring and simultaneously shifting towards the largest sprockets, especially if shifting several gears at once.

I've seen cases where the chain and derailleur can get jammed with the cassette here when derailleur is setup too close to cassette, leading to situations such as this.