I have found that humidity plays a big role considering an irrigation system around. If the water is hitting the foliage consistently, the theory of fungus play at hand. Most times in humidity and foliage contact, you will get burn that resembles plant shock. Smallaxe points out that the surface water may be fine but deeper roots are drying out. (A photo of the plant and the area would be helpful), however, I have noted that young trees-especially 3 gallon container grown plants are root bound upon purchase and often go through the plant shock period. At first, they look good......then defoliate.....then put back on foliage. Upon planting, I ruffle the root zone and apply upward slices in the roots to prevent compact strangulation. Then the other thing is aphid damage, thrip damage, and sometimes cotton scale. But to me......if foliage blow off is something that always occurs, then you may want to think about quick release fertilizers being applied if these trees are in the lawn setting. My last question is...............how hot did it get up there in Jersey this summer??? This plays a big deal with the problem with this tree.