Answer: Something is causing stress to your maple. Drought, heat, reflected sunlight? The prematurely aging leaves will drop from your tree and may very well be replaced with healthy new leaves later in the summer. If there were a pattern to the changing leaves - all on one branch, or concentrated on a specific part of the canopy, then I would suspect some damage to the roots or to the scaffolding branches. But, if the leaves are random in their placement on the tree, it's probably just a manifestation of stress and is self-limiting.
Best wishes with your landscape!
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