One reason I don't entirely trust kinetic pullers. I have one and have had no issues, but maybe some time. You are the first with ignition. I can't understand that because it takes a concentrated blow on the primer head to cause the nitrate to contact the anvil. That won't occur with any bullet puller, kinetic or not. BTW, The Grip-n-pull is a nice alternative as is a collet puller like Hornady's.
Obviously, no one has watched the old episode of Myth Busters where they cooked off a number of loaded rounds to see what impact a not chambered round would have... and they had basically nothing. The pill comes out and goes a small distance and thats it.
You have to 'contain' the charge in a constricted space that only allows forward expansion (into the bore behind the pill) for anything serious to occur....
You didn't, per chance on your backswing, make contact with something and the primer head?????