Black Locust tree

Question

Beginning of summer in central California, the tree is about 10 years old and is now dropping its leaves.

Answer 1

As the weather has warmed and the moisture decreases, increased water loss from the tree will cause them to drop excess leaves. This is their natural strategy to conserve energy.
If a spring is wetter then normal, this can make the issue even more noticeable.

If you provide watering and you see no other signs of disease or pests, there is no concern.