I have 10 mastic trees growing on my property in North Phoenix, AZ. All have put out lush new growth this spring since we had a rather rainy winter, but with the move into warmer weather all of them are showing fairly significant leaf drop. This is occurring among older growth branches with the leaves turning a bright yellow and eventually dark orange color. The newer growth seems unaffected and all appear to otherwise be healthy. Since mastic trees have a reputation for being "clean" trees with little plant debris, is this leaf drop normal?
Most likely the sudden move from winter to summer without an in-between precipitated the leaf drop. Just take care of your trees and they should recover.
Here's our article about Mastics: