I think you are saying that the brass was new when loaded, but the cartridges have been sitting around loaded for a few years. If I'm getting that correctly, I would have to rule out the brass and think your problem is either the rifle (possibly having a headspace issue) or more likely improperly sized cartridges that have had the shoulder pushed back, thereby creating excessive headspace.
You are correct, the brass was new from W-W, loaded by me, stored for several years.
I checked the head space of the W-W brass both fired and unfired, and found no difference in head space. Less than 0.001" difference. So the head space for the rifle and the brass is correct.
Head space on REM brass fired in the same rifle also showed the same head space.
Excessive head space tends to crate thinning just above the reinforced portion of the brass at the head. This weakened portion tends to fail with a circumferential crack due the the thinning. Not a longitudinal crack.