I haven't seen split necks in win 7stw brass shot as 7stw, but win hasn't made 7stw brass since '04 so some of it could be rather brittle.
I've got 200 win casings and shot all of them last year at least once, never loosing one. I have had a heck of a time with some twice necked 7rum brass I have though... the one batch must have not been annealed enough as I've lost 30 in a couple of firings...
Are you turning you brass to even out the neck wall thickness? Uneven neck wall thickness contributes to split case necks because the thinnest portion of the case neck gets increasingly weaker from one reload to the next and eventually fails. Turning the necks to reduce the uneven wall thickness helps prevent that from happening.
Brass was split once it was loaded, of the ones that split chrushed and could tell that the force needed was greater than one that I annealed. These were from last batch of stw brass Winchester made so more than likely quality control was lacking to say the least.