I had the same thing happen with my 270WSM, I thought I would just load my regular load and go to the range and rock it but when I fired the first couple I got a horrible tight bolt lift and the primers flowed into and filled the primer pocket flat.
What was happening was the new case would set deep in the chamber and at firing the primer would come back then the brass would slam back as it blew the shoulder out mashing the primer and putting a lot of extra pressure back because the case had no traction on the chamber. I think?
Fire formed the rest and then loaded the same load and had the normal stuff going on. It was a stout load and should have though it through but now I know and the wife loves shooting my fire forming load so it works better all way around.