Most of the other possible causes he lists are disease or pest related (e.g. bald brood, wax moth lavae damage).The Drawing in of Winter: At very nearly the same time [as the drones are ejected] several colonies remove drone pupae that are white, brown and almost fully developed to the hive entrance.
The last brood of drones still in the colony is being removed from the cells. (This can also occur in July). Colonies which have a strong tendency for egglaying, generally remove the drone larvae a few days later than the others.