Did you trim the cases before you started seating bullets?
Also back off the die until it does not crimp. Crimping is not needed on most rifle rounds, until you get to the hard recoiling large bore rounds, or are feeding a tube magazine fed rifle like a lever action. Also it tends to help accuracy a bit as well.
I would place my wager on one of two things. Possibly a little of both.
1. Brass not uniformly trimmed.
2. Over crimp
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.