Certified GKH Gardening Expert
It could be one of two things. The first is that the tree may not be getting enough water, especially if it's very hot. It should be getting at least 2" of water a week. Heat and drought stress is the number one reason for premature leaf drop this time of year, as the trees try to go into early dormancy as a means of protection when water is low. Up the watering and see if this helps any. The second is that it may have a fungus or pest that kills the leaves. Treat the tree with neem oil, which is both a fungicide and insecticide, just as a precaution.