Answer: Based on your description, it is a bit difficult to try to make a guess as to what is happening. The dry weather could certainly contribute to any type of stress such as insect or disease or the drought could simply be the cause. Since the branches are all on one side, and you don't see any obvious above ground cause such as pests or bark damage, in my experience it is a good idea to start investigating the root situation on that side. Sometimes there is a difference in the drainage pattern so the soil there is a bit drier, or if you irrigate the irrigation system is problematic, or there is a rock underground causing an obstruction to root growth, or there is an encroaching tree root from another larger tree, or there is extra de-icing salt runoff from an adjacent patio that has affected the soil, or something like that. Did a vole attack the roots? Another possibility since it is on one side is accidental lawn herbicide or even spray overdrift (or spraying at high temperatures or a concentration or a material maybe not suited to the maple -- any time you are spraying make sure you read all of the label instructions and follow them exactly) or similar one time cause such as soil compaction due to construction or vehicle traffic. Next, is it possible this side has sun scald damage to the trunk or evidence of old ties that held the tree to a stake at one time or even weed whacker damage? This type of old damage can take a long time to become evident as a problem in the branch growth pattern. Was there anything unusual in the mulch on that side? Finally, the west side tends to be hotter and sunnier due to the afternoon sun. If the soil has been allowed to dry out, this could be enough to make a difference, these trees are known to prefer an evenly moist yet well drained soil and do suffer badly in drought -- although I would have expected it to show at the top of the tree rather than near the bottom. Essentially you are trying to play detective and seek out anything "different" that has happened or could have happened.
At this point I would suggest making sure the tree is well watered this fall by rain or by the garden hose, enough to keep the soil evenly moist but not sopping wet beneath the mulch until the ground freezes. If the soil is dry, you might want to water deeply, wait a few hours, then dig down and see how effective the watering was or wasn't -- it can be surprising. Then dig down to see if and when you need to water again. (In cooler weather, you may not need to.) If the damage continues to spread, I would suggest you consult with your county extension or perhaps a professionally trained certified arborist who is familiar with fine ornamental specimen trees and see if they have any suggestions. Good luck with your tree.
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