Answer: A typical reason for a newly planted tree to defoliate is dry soil, especially if it is combined with excessive summer heat. This tree will need an evenly moist soil while it is becoming established; about the first three years or so. Once established, it becomes more tolerant of average and dry soil. Drought or dry soil while it is building its root system would cause stress and consequently cause it to drop leaves and look droopy. You might want to check the soil and see if it is moist or if you need to water. Dig down a few inches with your finger, checking both the original root area and the surrounding soil. When you water, water deeply and slowly, wait a few hours, then dig down and check to see how effective your watering was. It can be surprising. Also use several inches of organic mulch over the root area but not allowing it to touch the trunk. There are also some possible pest and disease problems that could affect this kind of tree, although they are not so common. Since your tree is new, I think it is just having a stressful period of adjustment. Water deeply once a week (or more often if soil moisture retention is poor) and allow your tree to adjust to its new home. I'm sure it will and next spring it will leaf out and look healthy.
Best wishes with your landscape!
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