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Rocky's explanation is the first one I've heard that makes any sense regarding this phenomenon.
I can't argue with the observed behavior, but I've never bought into the theory that the powder magically knows what the maximum rated pressure is for a given caliber and starts behaving non-linearly at that pressure. The same powder might be used in various cartridges with max pressures ranging from 50K to 65K.
What Rock wrote makes a lot more sense. However, it seems to me that if excessive obturation is causing this behavior, pressure must be very high indeed, and/or the bore must be rough. The pressure at which this occurs need not have anything to do with the max rated pressure for the rifle/cartridge, and is probably above the max rated pressure for ALL cartridges.
Also, it seems a lot would depend on how hard the bullet is. Maybe you would not see this phenomenon at all with a solid copper bullet. Not that it can't obturate, but you would be seeing unmistakable pressure signs and/or velocities long before you reached the necessary pressure. Maybe.
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