Why do leaves change color and fall off in Autumn?

Photosynthesis.

A standard answer derived from experience in northern areas does not explain why the same species of trees continue to lose their leaves in winter even when the tree is in Venezuela or Cuba. There are eminently good reasons why trees should lose their leaves before snow burdens them, and the trees may respond to a number of signals to tell them when to ditch the leaves. But a simple explanation like declining sugar production does not account for the action in the tropics. In Cuba, leaves do fall later than in Ontario, and they are budding out right away after the leaves fall. My guess is that the trees plan budding, and budding pushes off the existing leaves in the tropics. But that does not explain timing.