It happens, just toss them, or recycle them. Pressure, stress, Quality control during manufacture, any number of things. I just do a quick visual on mine before I prime them. Any cracks---> reject bucket. Any cracks after firing when I pick them up , tossed. It looks more serious than it is. I keep reloading my cases until they do this, that tells me their life is over.
"Socialism is a philosophy of failure, the creed of ignorance, and the gospel of envy. Its inherent virtue is the equal sharing of misery." - Winston Churchill