Rich,
Anytime the servo is being called to go somewhere it doesn't have the power to move to or even hesitates even momentarily anywhere along the travel or even loaded to point where motor isn't able to run at full rpm. The motor is being fed juice to go but it can't go so it doesn't move and the locked rotor or stalled servo occurs. When this happens the amp draw goes through the roof and you have very close to a dead short at the motor wires. This heats everything up really fast and the load is more than the electronics were ever designed for and it smokes the servo. Often it is a part on the board callled the amplifier that releases the majic smoke but when they lock up any part including he motor is suseptible to being fried. If it doesn't move for a few seconds that is all it takes.