One way that you can get to this type of failure is to shoot light loads with a rimless case. If this is progressing, you would see increasing protruding primers as cases are repeatedly reloaded. If at some point you shot a hotter load, the case would be stretched, and the primers would be flush. Another way that this hot load pressure can be created is that the case necks can become too long, as the bodies get shorter, so that eventually they start to jam into the bullets as the rounds are chambered. This gets a little tricky because if you are only measuring case length, and not using some sort of gauge, at the shoulder, the fact that the necks are too long may not be noticed, since the bodies shortening will combine to keep the case lengths within limits.
On the other hand, If you pick a more normal pressure load, the primers will be flush, on every firing, but it is more likely that you will progress to needing FL sizing sooner.
What happens with light loads is that the firing pin strike, and the force of the primer on the bottom of the primer pocket knock the case froward in the chamber. The Case then grips the chamber wall with the pressure of the powder's burning, but the pressure level is not high enough to stretch the case back to the bolt face. Repeatedly done, this progressively shortens the case from shoulder to head, and lengthens the neck. It may not be apparent what is happening, if the cases are kept trimmed, but you can see it is taking place if you use some sort of gauge on the brass, or you may see that the primers are above flush on the fired cases.
Case designs with sharper shoulders are less prone to this sort of problem. The other side of the coin, for cases like the Ackley Improved, is that their primers are hard to read for pressure. The extreme flattening that shows up at high pressures with more normal case configurations happens in the interval that the case head is forward, and as it is stretched. With more extreme shoulder angles, cases have greater resistance to being knocked forward as they are fired.
When reloading for a Swift (smaller than average shoulder angle) I found that I needed to set the FL die so that cases "headspaces" were as they came out of the rifle after firing. Any shorter, and the additional room and the case could "take a run at" the chamber shoulder that allowed it to be driven farther forward as it was fired, and a bright line would show up on the next firing. I was using medium loads that had the primers flush. If I sized so as not to bump, but only maintain the same clearance as the fired case, the problem did not happen, and case life was good.
With more normal pressures, the variable may be how soft the shoulders are. Everything else being equal, thicker, harder brass will not as readily allow the case to be driven forward as it fired, so less stretching per firing will take place. This does not augur well for those who anneal cases that have relatively smaller shoulder angles. While gaining in one area, they may be losing in another.
