I've saw this with unfired new ammo before 3 out of a box of 20 were split when purchased from the factory.
I've occasionally found a split case neck on an unfired piece of brass right out of the bag, and occasionally after the first firing.[:@]
That's the unfortunate thing about a lot of the remchester-short-whatever magnums- most of the time you are stuck using brass of inconsistant domestic manufacture since you can't get a lot of them from Lapua or Norma.