Are you neck turning? If you go too far into the shoulder with the cut, this will happen. Taught a buddy how to do it once, as he was borrowing my rifle, so I wanted to spread out the labor. I obviously didn't make it clear just how far down to go...
About 1 in 5 pulled off in the sizer, and of the remaining, about 1 in 10 pulled off after firing.
I tried to quarantine and dispose of the bad ones, but every so often, a neck comes off in the chamber. I think they're all gone now though.
Invest $20 in a
universal decapping die. I wouldn't want to be running dirty cases into my dies. At the very least, vigorously wash the brass in warm soapy water (and dry of course) before you start the sizing process.