I'll tell you what an engineer at RCBS told me about that about a gazillion years ago. At the time I was playing with a .240 Wby and having a lot of fun burning up the barrel. And I too noticed that I was getting neck splits right at the upper radius. So I too started annealing (something I hate to do) and I got something much more normal from my brass.
Well about a year later, I went up to RCBS for who knows what now, and I talked to one of the engineers. He said the double radius causes the brass to flow less gracefully during forming and that they get a little work hardened right from the factory. Well, that's what he said. Sounds plausible to me, but my recollection tells me that case neck annealing is part of the manufacturing process. So if the cases were work hardened at the neck, it should have been annealed out. But take this all with a grain of salt, I don't trust my memory all that much these days.
