"Regular" bottleneck cartridges headspace off the shoulder (firing pin pushes the case forward till it stops), then when fired the case stretches backward until it touches the bolt face. So if your body or FL die tends to knock the shoulder back too much, it will stretch the brass near the base with every firing until the case head finally separates.
What you showed in the picture looks like a belted Magnum case. Those are supposed to headspace off the belt. But again, if the chamber is too generous for the shoulder position, and your die knocks the shoulder back after every firing, it will only be so many cycles before the case head separates.
There is something wrong between your chamber dimensions and your die dimension to cause that problem. I don't own any belted magnums nor dies for them, but I think you need to increase the distance to the shoulder on your brass so that it does not need to be stretched with every cycling. It is certainly not the kind of failure mode I want to be having with my brass....
OK, below is the 8mm Rem Mag which apparently is the parent case.
If the dimension E (5.59mm) or L1 is off, it will create the problem you are experiencing. Have you ever bought a head space gauge to check ? $30 to save your brass ?