There are several reasons why maples drop leaves in the summer. Summer drought, a petiole leaf borer insect or too much water.  Maple trees will also drop leaves if the canopy is too full. This type of leaf drop helps more sunlight reach the interior of the tree. It sounds like the older leaves are dropping on your tree so I do not think it is any serious insect or disease problem. My best guess is too much rain. The nice thing about deciduous trees is they get a fresh start every spring so I suggest you wait this out and see what happens next summer. Keep growing, Marianne Binetti