My first guess would unfortunately be verticillium wilt (Verticillium dahliae). Hopefully somebody else will have better news. :)
Verticillium is very common with maples, and nearly impossible to control, though not always fatal to the tree. It typically starts out with spotty leaf death across the crown, progressing to entire branches dying out as the vascular system clogs up.
If you find a branch that has a fair amount of dieback on it, and cut it as a angle you should find black discoloration in the wood. Control is sometimes possible by cutting off the branch back to where there is unaffected wood. I find it's helpful to sterilize my pruners between cuts so I know I'm not transmitting the fungi. A 1:9 bleach to water ratio works well.
I'll leave you google further info on it.
As a second guess, would be lack of water as it looks like you're on a slope. I often find it's helpful to make a dam on the downhill side and form a basin around the tree, about 18" from the trunk and 2-3" above the crown. That way I have a nice basin to fill with water.