Case head separations are especially common with belted magnum calibers. Factory loads headspace on the belt, but handloads MUST have -.002" or less chamber clearance at the shoulder.
This shows how case separation begins inside of the case. Reducing chamber clearance (at the shoulder) prevents the case from stretching (and thinning) the brass. Full length resizing is the way to go, but your die height MUST be set accurately, and measuring is required.