primers not seated tight or excessive FL sizing

I would say lube on primers if they hadn`t fired second time,,,could be a thin lot of primers also,,or some brass thats been overloaded,,I`d try it again with different bras and primes..

Your lucky he didn't dirt roll you.

Likely. I'd also say firing pin dragging but it wasn't that cold this afternoon. Mr. big's probably right. I vote for oversizing. I like to have them where I can just feel a little pressure when closing the bolt so I know I don't have that little bit of headspace that can cause this problem. My namesake the 35 Whelen is notorious for this problem and I have to watch it like a hawk. My Ruger and NEF both have long chambers and you can't let the dies full length size and bottom out the press on the die or you'll get a nice click when you pull the trigger.

Take the firing pin out of the bolt and then try the brass to see what the fit is. It'll be a little safer that way. You'll still have the drag of the ejector spring but slowly adjust that die in until you can just close the bolt on a sized case. It's worked for me in the past. Good luck.