Unclenick give good advice above. I'll add that even if the crimp isn't excessive, starting it before the case mouth is lined up with the cannelure can cause this problem. If the case mouth is squeezed into the bullet as the bullet is being seated the neck has nowhere to go but down.
I don't crimp anything I reload for my M1.
Proud member of the NRA and Texas State Rifle Association. Registered and active voter.