Are there any particular symptoms on the leaves themselves, like spotting (not necessarily a serious issue), deformities, or other conditions? Maples as a group are not highly drought-tolerant, and lots of mature trees are suffering and stressed by our ongoing drought coupled with high heat. Leaf shedding is one way plants reduce their water demands when soil moisture is insufficient. If possible, try to irrigate the root zone of the tree periodically so the roots remain healthy; tree stress sometimes has consequences for long-term health that might not manifest for months or even years after the original event (like drought), so stress prevention or reduction can be key in helping to support tree health and longevity.

If you are unable to water it due to water restrictions, limited resources, or budget constraints, then unfortunately there is not much that can be done, and the tree might just go dormant early this year and have poor fall foliage color this autumn (if it retains enough leaves by then). If you suspect other issues are causing leaf drop, like girdling roots, trunk injury, or any of the other causes of tree decline, then see if a certified arborist can come out and assess the tree in person.

Miri