I had that same experience with a bag of 7mm Rem Mag brass I bought last year. New brass in a factory bag, and for the life of me I can't remember if the brass was Remington or Winchester. I prepped it all, like I do new brass and on the first firing a considerable percentage (about 30%) showed splits in the shoulder. I chalked it up to bad annealing at the factory and didn't make an issue of it.
.30-30 brass has that long, lovely thin neck that cast bullet shooters so admire and I suppose it's possible that a bad batch gets out once in a while.