It was my understanding that powders expect a certain amount of containment or "pressure resistance" as they burn. If they don't get this they can basically explode instead of burning real fast as they're supposed to do. This is my understanding of the theory of detonation due to too low a charge.
No, that is incorrect. Without any containment, modern progressive Smokeless powders do not explode, they burn much slower, with a lot of sparks and flames, but they do not explode. Progressive powders need containment to operate properly and generate pressure. The more pressure they generate, the faster they burn.
Here is a completely different theory, the one I subscribe to.
The theory is called Premature Shot Start. It has nothing to do with powder position.
Most powders require around 5000 psi before they begin burning consistently. Under 5000 psi they can burn in fits and starts, they can even go out. But far less than 5000 psi can push the bullet out of the case. Just a primer can push a bullet out of the case.
The theory goes like this. With a very small powder charge, the powder may not get a good burn going. It may start burning, and develop enough pressure to push the bullet out of the case. But once the bullet has moved, the internal volume of the burn chamber (the inside of the case plus however far the bullet has moved) may have increased significantly.
We all know that a well balanced powder charge will have enough powder in the case so that even as the bullet moves down the barrel, and the volume behind it keeps increasing, the powder will continue burning fast enough that pressure will continue to rise, despite the increasing volume. But in some cases, if the powder charge is small enough, and the pressure drops a whole lot below 5000 psi, the bullet may actually stop moving a short distance down the barrel. Once this happens, if the powder is still burning, albeit fitfully, the burn chamber has suddenly become finite, it is no longer expanding. So the burning powder will continue to generate exhaust gasses. This will in turn raise the pressure.
As I said earlier, Smokeless powders burn progressively, in other words the more pressure there is, the faster they burn. In the case I have outlined, the stuck bullet will at this point stop acting as a projectile and it will instead become a barrel obstruction. Pressure will rise rapidly as the powder continues to burn progressively behind the barrel obstruction. Pressure may rise so quickly that the bullet literally cannot get out of the way fast enough. Remember, it's not like when the bullet was back in the case, and pressure started building, this time pressure builds much more rapidly and the inertia of the stuck bullet keeps it from getting going fast enough. If there is enough powder left to generate enough pressure, a burst chamber can result. This would all happen so fast just a microsecond or so, that the shooter would not be able to detect that the bullet had stopped before the gun blew up.
We used to talk a lot about this in Cowboy Action Shooting circles, where a lot of guys shoot very light loads. There has not been much discussion of it lately, but this is one reason it is always recommended to use a heavy crimp with light loads, to retard the bullet leaving the case a millisecond or so and allow pressure to build enough that the bullet keeps movinig.
In one of his books Mike Venturino writes about blowing up a fine old Colt one day with some light loads. He went back and pulled every bullet in the lot, and weighed every powder charge. He did not find two bullets in any case, and he did not find any double charges. It is entirely possible he somehow managed to double charge a single case, but Mike is not careless, and I really doubt he did so.