I suspect a rat, or mole or mouse, has burrowed into the rootball and is causing root damage. The clues to the mystery are that the trees nearby are still green so it does not sound like a disease or insect invasion. Also the leaves are turning color rather than immediately dropping while green which means there are still some roots active. Check around the base of the tree for holes in the ground. Not much you can do except set a trap, water, and mulch the soil. Often the damaged root system will grow back after a year or so. Good luck and keep growing, Marianne Binetti