Why are the leaves on my tree turning yellow?

If the yellow leaves are coming from the inside of the tree, it is likely responding to a change in soil moisture. Trees put on an abundance of growth in the spring when moisture is high and then find they can't support all that growth as the weather warms and the soil dries. They will shed excess leaves to conserve moisture. That may be what you're seeing, but it is best to make an appointment to have someone take a look.