Could be several things......was the brass full length resized or neck sized?........Were the cases trimmed to minimum length? Was cartridge over all length exceeded or the bullet seated farther out than specified? How many times has the brass been loaded and was the neck thickness checked?
Not knowing the details of how the ammo was loaded it's difficult to tell where the problem might be, or if there is a problem at all...........can you give us more specific information about the load?