Certified GKH Gardening Expert
Both over- and under-watering can cause leaf yellowing and leaf drop. Vary the water you provide according to the heat and humidity of the week, and don't let the tree dry out until the leaves wilt. Keep in mind that trees in containers need more frequent watering than those planted in the ground.
Other types of stress could cause the same symptom, such as exposure to excessive fertilizer, salts, or toxins.