This is not a mechanical failure on part of the firearm, reloading equipment, nor the powder itself. This is a reloader pushing things to the limit and not wanting to take a systematic approach to the reloading with the proper precautions.
It happens... It's learning from failure.
I've reloaded hundreds of thousands of pistol rounds that took less than 50% of the case volume (9mm 147gr bullets, titegroup powder). You look in each case, every time. I have yet to have a double charge or squib with this load, and only one squib ever. I caught it before it left the machine.