If I read your post correctly, it would seem you said your ammo that was splitting was brand new Winchester brand factory ammo. Like Mike said, it could be bad heat treating by the factory, but I never came across anything like that from Winchester ammo.
If you have had this rifle for many years and never had a problem like this before, then I would suspect the case quality as Mike suggested. However, if this rifle is new to you, and if these cartridges were the first you fired in it, I would be concerned about the chamber and the headspacing of same. If there is a problem with the rifle, splitting new cases can be a warning sign of a potential danger waiting ahead.
If the rifle is new to your use, a trip to a competent gunsmith to inspect the rifle and cases may be a good idea.
Keep me in mind when you first smell the new mown hay this spring.