That isn't firing. Certain groups or classes were allowed to leave when their term was up. Some categories had their time shortened, but that isn't the same as firing.
What do you think it is when you still have time left on an enlistment and you are informed that you are being discharged? You're fired just as certainly as the Chiefs and Petty Officers were when I was in the Navy and the Navy told them they were being forced to retire to make room for younger men to move up and remain in the service.