You don't mention if this is a young tree planting or a mature established tree.
Yellowing leaves can indicate a watering issue; too much or too little.
If the tree is new, water frequently enough so that the top few inches of soil are moist but not soggy. Crumbling soil is too dry, soggy soil is too wet.
If it is a mature tree make sure the soil is well draining.
Hardwood mulch can help with moisture retention if the soil is drying to quickly.
You should always inspect the tree for any infestation to rule out that issue.
If the watering issues are resolved, but leaves continue to yellow, you may want to have a soil test performed, as this could be a nutritional deficiency.