Can I ask how the cassette is being used?

Its not a performance cassette and you cant just shift and forget. I havea PG-850 and you have to be careful shifting these things especially when the chain is under tension up hill. There are times I'll have to jump off the bike and walk up a trail hill rather than forcing it into a granny gear and stressing it.

You have to use consientious shifting with these cassettes as they don't seem to be able to take the stresses of rough shifting techniques.

Keeping the gear train clean and well lubed is another thing that could help.

The SRAM cassettes I have are not too forgiving for rough shifting and am always thinking of ways to relieve the stress of the drive train in my rides.

I think that pic by Chelboed shows a hub under immense standstill torque were one would be on a steep hill practically motionless while torqing the crank with some strong legs might I say. Maybe over and over causing fatigue failure.
Or the cassette fit on loosely (poor tolerances of hub or cassette) causing a lot of movement between hub and cassette mating.
Or a hub whose metal was too soft.