This is a common cause of concern, but it doesn't have to be.
Trees lose a lot of moisture through their leaves.
When stressed by the dry summer, the excess leaves drop. They drop, but it's all natural. Leaves that have been invaded by diseases or insects can drop for lack of light or from mechanical damage. This often happens in Montana toward the end of summer. Because the season is near a close anyway, you don't have to spray for the pests.