At a spot where the stem of each leaf joins the twig, a thin ring of cells called the abscission layer begins to form. This layer gradually covers the base of the stem, cutting off nutrients to the leaf. As the leaf stops making food and the chlorophyll fades, sugars trapped inside the leaves react with the sunlight and other leaf chemicals to reveal new colors. In some trees, like the oak, one side of the leaf gets enough sunlight to reveal new pigments but the other side doesn't. This explains why oak leaves are often dark brown on the top and light on the bottom.
Weather seems to play a role.
Local foresters say heavy rainfall and high humidity during the summer can encourage fungal growth and lead to early browning of leaves. Of course, not all trees lose all their leaves every fall - spruces, pines, hemlocks, and firs keep making food all winter. Their needles are really just thin leaves that don't fall away every year. But evergreens aren't really green forever. Every fall, most evergreens shed their old, dying needles - we just don't notice because the young needles remain green.