It could be “maple decline” more on that HERE. As you can see from the article, it could be related to the last 2 very harsh winters. If that is all it is, it should bounce back. You should also check to make sure that you haven’t been piling mulch up against the bark of the tree. While this seems like a nice thing to do, it is just the opposite. Bark, dirt or other material piled against the trunk slowly cuts off the flow of moisture and nutrients from the roots to the branches and leaves above. It will cause the symptoms you describe and will eventually kill even a full-sized tree. Brush any mulch away from the trunk until you find the original soil and leave it that way. Mulch is good around the tree, but not piled up against it.