They need to save up their energy,so they lose chlorophyll(what makes them green) so they won't need to use energy. Now,chlorophyll is what makes food. No food, No leaves. They drop!
leaves fall out of trees because it is too cold for the trees to grow and be healthy. they do not get sunlight; and you know that trees need sunlight to live and grow, so the leaves fall out. but...
During the season of autumn, also called fall.
More detail:
As the temperatures get cooler, and the days get shorter, the tree sap goes down. The changing color of the leaves is due to reduced...