R5R and Versifier have the problem nailed for you.....its all in the crimping.
Just be aware that with a roll crimp (which is what I use) the case length is critical. Any variation in case length will cause more or less pressure during the crimping process. I would bet that your cases with the distortion were longer in length than those without the distortion. You might check your case lengths. You might also want to invest in a taper crimp die.
BP | Bronze Point | IMR | Improved Military Rifle | PTD | Pointed |
BR | Bench Rest | M | Magnum | RN | Round Nose |
BT | Boat Tail | PL | Power-Lokt | SP | Soft Point |
C | Compressed Charge | PR | Primer | SPCL | Soft Point "Core-Lokt" |
HP | Hollow Point | PSPCL | Pointed Soft Point "Core Lokt" | C.O.L. | Cartridge Overall Length |
PSP | Pointed Soft Point | Spz | Spitzer Point | SBT | Spitzer Boat Tail |
LRN | Lead Round Nose | LWC | Lead Wad Cutter | LSWC | Lead Semi Wad Cutter |
GC | Gas Check |