I have a Ornamental Japanese Maple 'Roseum Ornatum' (Acer palmatum) that has been losing it leaves in abundance since early June.
Since we have had a lot of rain this spring, there may have been excessive rain water on the ground where the Japanese Maple is planted resuting in leaf drop. If it is a well draining site the next thing to suspect is anthracnose. This fungus will cause rotting spots on leaves and maybe stems resulting in possible leaf drop. One year of leaf drop is not of great concern especially since the tree is growing new leaves. Good clean up practices in fall and winter will lessen the reoccurance of the fungus. Repeated leaf drop over a number of years will cause the tree to deteriorate. Treatment with a funguscide containing mancozeb is the recommended treatment. The disease causes light brown spots on the leaves. You can have the leaves checked at Cooperative Extension in Doylestown for antracnose.