I can appreciate that you are distinguishing a shock wave from pressure, it still does appear that the phenomenon tends to happen in large cases loaded to relatively low pressures.
I would change this to read, it still does appear that the phenomenon tends to happen in large cases loaded to relatively low pressures when less than ideal propellants and questionable loading practices are used.
...and before jumping all over me, this is not aimed at anyone, I include myself in that category. I don't think you'll find a powder company or loading manual publisher that would consider stuffing a varying amount of Dacron, foam, cream of wheat, or dryer lint in a cartridge as a recognized loading practice. We don't do it because we want to, we do it because we need to if we want to duplicate original black powder ballistics in many cartridges that werent designed for smokeless powder. The proper powder to use to achieve factory ballistics would be black powder. There's a reason you see powders with names like 50BMG and CFE223. They were designed to have the ideal density and burn rate to work in those cartridges. We don't have a smokeless propellant that mimics the density and burn rate of black powder in all situations so we have to make do with what we have and with that comes risk.
The question would be, why doesn't this condition happen in what are commonly known to be high pressure cartridges. Automatic and semi automatic actions would seem to be dangerously vulnerable to feed and extraction problems, but it doesn't seem to happen?
If we look at modern rounds like the 308 and 223/5.56 they were designed to work with smokeless powder. Their target velocity is achieved with powders that fill the case and work at target pressures. If we were to neck down a 300 Ultra Mag to 22 and try to mimic 223 ballistics we'd run into the same problem. There just isn't a powder that will allow us to easily achieve that without risk.
As an aside, while the dicussion is about ringed chambers, I would think it also relates to some blowups.
Kinda, sorta, maybe???
Again I will do my best to oversimplify this. Please don't jump on me if things are not totally accurate. Excepting obstructions, blowups are generally going to be a result of using too much powder. This will be more common when fast powders are used as less is needed to reach high pressures. In some cartridges two or even three times the max charge can be placed into the case. This is less of an issue with slower powders as more is needed to reach the same pressure and you often run out of space before you can reach a pressure high enough to blow up an action. That said this will apply more to the individual that uses Red Dot in a 30-06 rather than the individual that goes two grains over max with a charge of Varget. The latter is certainly bad but is less likely to result in a blowup.
Smokeless powders are engineered. Theres single base and double base. Theres flake, ball and extruded. Some grains have holes in them others dont. Some grains are bigger than others. All this is done to modify the density and the burn rate to allow it to perform optimally in a cartridge (or several cartridges). This engineering is almost always done to slow the burn rate. Fast is easier to achieve.
Why do we care about burn rate? As I mentioned before its not just the pressure we care about its how fast the pressure is created as well.
If I place a car in neutral on flat ground and run into it at 10 fps with my 200 pound body weight I will exert 311 pounds of force on the car. If I assume my arm weight 10 pounds and I slap the car with it at 50 fps I will exert 389 pounds of force on the car. Which one will move the car further? Which one will hurt more? I hope most will agree (otherwise my example sucks) that the first despite being less energy will move the car further and hurt less. Again its not just the total energy its how and when the energy is released. Another example is in mining. Lets assume we are quarrying granite and long bore holes are drilled in the rock. If those holes are filled with a slow explosive the granite will separate in large chunks. If a fast explosive is used you will get stone dust. The total pressures can be made to be the same but the results will be different. This hopefully helps you to understand why we choose one powder over another.
If we use a very fast powder in a cartridge it is possible to raise the pressure so fast that the action fails before the bullet can be expelled. Thats the reason we cant use high explosives in guns. In a perfect world the propellant would start burning slowly until the bullet began to move and then rise to max operating pressure and stay there until the bullet just leaves the barrel where it would be totally consumed. Unfortunately they dont work this way. They usually work just the opposite and reach peak pressure quite rapidly and then drop off as the powder burns and as the internal volume increases (due to more barrel length being available as the bullet moves forward). The powder manufactures try to make a powder that will fill the case and slowly and over time push the bullet forward so the chamber pressure stays in the target range without going over. So why dont we just use slow powders for everything? A byproduct of modifying/slowing the burn rate is that those powders require higher pressures to fully combust. If the case volume is too big and the projectile too light the powder will not burn completely (if at all) and you often get a bullet stuck in your barrel. In many pistol cartridges the pressures needed to ensure a good burn with slower propellants are higher than the pistol can withstand so you need to go to a faster powder to get it to burn at the lower pressures. This is why there are so many different powders and why not all work in every situation (cartridge). This is why high volume cases (usually originally designed for black powder) require faster powders and have more room in the case than needed. This room allows for the potential of double charging (with a powder thats less forgiving) and these fast powders and air space can also lead to the chamber ringing. I guess they are related in that way but the reasons they happen are different and the mechanisms that create them are different.
