I have a 20+ year old maple tree that has been healthy. This summer I'm seeing branches with leaves changing color on the edges (yellowish) which progresses to the full leaf(leaves).  It seems to be mostly on the branches that get the most sunlight.  This process is spreading from lower branches to upper branches.  I first noticed this about 3-4 weeks ago. 

The tree is in a yard that has grass under about 1/2 of the tree.  The remaining area is open soil with flowers and shade loving plants.  The grass is watered 3 days a week in two sessions about 15 minutes long. The flowers are selectively watered via dipper irrigation. This has been the watering regimen for at least 10 years.  The soil is clay but it drains well on the surface. There has been no disturbance of the roots. We fertilize the plants but don't use any weed control preferring to hand clear weeds from the grass and flower beds. 

I do have squirrels in the area but I can't tell if they're nibbling through the bark on the problem limbs.  I did lose one small limb due to squirrels weakening the limb.

I see lots of trees yellowing and am assuming it's the result of the hot summer.   Could heat be the problem?