The small amount of powder does not have sufficient mass to cause the burning rate curve to take the 'normal' amount of time to max pressure. The burning rate curve instead is greatly accelerated to max pressure in far less time than normal for that cartridge design. The result is that the bullet does not have enough time prior to max pressure being reached to get moving down the barrel relieving pressure as it moves. Ideally the bullet should be nearing the end of the muzzle at max pressure. Instead the bullet becomes a 'plug' with no movement in time to relieve the developing pressure. In effect a miniature explosion occurs instantly within the cartridge with a resulting overpressure to the cylinder walls. Certain powders are more likely for light loads to detonate than others. ........... Big Cholla