Mike,
Case life is reduced when case lengthening occurs.
This can happen even if the rim thickness and headspace at the rim is correct.
If you are pushing the shoulder back in the resizing die, and then the shoulder is moved foward when the cartridge is fired, then it gets pushed back again during sizing, and so on and so on, this will reduce the working life of the case and can cause head seperations.

Other issues which can lead to head seperations include FL dies which reduce the dimensions of the case wall diameter/s in relation to the chamber dimensions, and double rifle actions going off the face and as a result increasing the headspace.

Never never never anneal any part of the case other than the neck and maybe the shoulder if necessary.
Brass cases need to be of a certain hardness at the head to be able to contain the pressure.
Serious accidents due to soft brass have occured when the case head has been softened as a result of being heated and the case head has expanded and released hot gas under high pressure.