As the sunlight hours diminish in the fall, and the quantities of nectar and pollen coming in to the hive decrease, these factors become clues for the queen to decrease the quantity of eggs she lays.
The decreased size of brood nest allows for the populations of the beehive to slowly diminish in to the fall and also increase the spacial access for pollen and honey storage in the brood nest locations without the necessity of building more wax or expanding the hive size.
This is a very important part of fall because the colony must be at just the right size for the amount of honey stores available to the colony as well as large enough to keep the colony warm throughout the year. Too large may cause them to starve out, and too small may cause them to freeze to death. This is the important risk that the beehive is concerned about when entering the fall!