Maples will sacrifice leaves here and there
during times of stress..
Heat, dry weather, not much food for it's roots, etc...
They will selectively choose leaves, to stop
feeding and they will dry up...
That is somewhat in the same process it takes
when turning to the fall colors too...
Your's look like its' just changing color for the season... Cooler nights and lighting *where the sun is* start the color changing
process for maples...and drier
growing conditions.
Here's a bit more of the technical terms too.
Brief overview of a tree leaf during autumnal color change