Having the decaping rod adjusted to far down causes this. It bottoms out on the bottom of the case and causes it to bend. With the amount of leverage a press gives you it's likely you never realized it bottomed out. I had a forester FL sizer do that to me. It couldn't get it to bump he shoulder back and wasn't paying attention as I adjusted the die further down I didn't adjust the decaping rod up. Never was able to get that forester die to bump the shoulder either
Advanced Precision Rifle Reloading
High-level technical discussion on precision rifle reloading techniques, equipment, & components. Not for casual handloaders.