What causes drone bees to die?
The number of drones in a colony may be hundred-thousand , i.e 15 % of the total population. The drones lack wax glands, pollen collecting-appartus and sting which is present in worker bees which make them completely dependent on the workers, transferring its genes to the next generation is the only contribution of the drone bees. Usually the drones do not mate with the queen of the same hive. The drones that are able to mate with the queen die shortly after mating due to breaking of bulb of endophallus(penis) inside the queen. 



The rest, the survivors of the mating flight (mostly drones that could not participate in  mating) are not allowed to reenter the colony. They remain in the vicinity of the hive and eventually starve to death.

In hours of crisis when the colony needs to drop the expenditure of resources(food) the workers eat up larval drones or drive out the matured drones from the colony that eventually die due to the same reason.

Information taken from:


Wikipedia
Bees
The Backyard Beekeeper