In response to the shorter days and diminished sunlight of early autumn, a layer of cells forms at the base of each leaf where its stem connects to the tree.  This layer gradually closes the veins that carry fluids in and out of the leaf.  These clogged veins trap sugars in the leaf and promote production of pigments called anthocyanins that give the leaf color.  Once this separation layer is complete and the connecting tissues are sealed off, the leaf is ready to fall due to wind, rain or gravity.

Adapted from U.S. Dept. of Agriculture