If you haven't watered lately you should. I notice my maples being mighty sensitive to heat and drought.

I have a client that has a maple doing the same. Found out the sprinkler has only been hitting part of one side of the area around the tree.
Like other Maples the root structure eminates outward in a large circle around the tree and you need to water around a large area to make sure you soak feeder roots that may be up to 6-8 feet away from the trunk of the tree. Pouring water at the trunk is never gonna work except for the smallest of trees.
There are also several kinds of fungal diseases that affect Jap Maples causing root rot wood rot and leaf spot. I would be willing to bet it's simply a lack of water unless it is on routine irrigation.