Why do I keep breaking rocker arms?

About 8 years ago, I bought from a closing dealership what was advertised as a NOS set of Slant 6 rockers and shaft (all assembled) for my '72 Valiant. I questioned the "NOS" claim because of the box (I don't remember the aftermarket name that was printed on it ) but I was blinded at this "Great Find". Anyway, I took the whole shebang apart, cleaned and reassembled for installation. I installed it and within a minute of startup one of the rockers broke just ahead if the circle that rides on the shaft. Thankfully the broken piece didn't fall deep into the engine and no other damage was done other than the broken rocker. This was on a stock solid lifter cam engine with proper lash. Could there have been a bad batch of rockers? or was mine weakened/rusty from within? I took the whole assembly off and set it aside. I never really inspected deeply but still have the setup. This was my experience.
Hope you get yours sorted out, Good Luck!