What causes my tree is dropping leaves in july like it's already fall?

Physically the leaf is cut loose at an abscission layer at the base of the stalk, the same mechanism as autumn, triggered early by stress signals instead of daylength. What pulls that trigger varies. A wet, cool central Indiana spring produces heavy anthracnose and scab infection that shows as drop in early summer. Midsummer heat and drought on clay soil that has shrunk away from the roots produces drop in late July and August. Roots that were cut, compacted, flooded, or left circling at planting produce it in any year that adds any further stress. Fungal or bacterial invasion of the water-conducting vessels produces it branch by branch. Two or three of these commonly stack in the same tree, which is exactly why diagnosis from a photo fails.