An alternate explanation is that the primer explosion pushes the bullet into the lands, jamming it in place. This causes the burning powder to not have the normal expansion space because the bullet is not moving forward and the force of resistance is greater on a stationary object than a moving object.
This causes a "plugged bore" event and the pressure level in the cartridge increases to detonation levels, and then boom.
The exact causes of SEE aren't as important as ensuring the conditions that are known to cause it are avoided.
Machine guns are awesome until you have to carry one.