Trees is water stress, Leaves asked for water and roots was not able to provide them, It dropped all leaves to reduce water loss.
Its feeder roots are too weak or Damaged, and It takes time to recover, roots love airy moisture soil but not over-wet soil, it grows the best at Temperature between 75F to 85F range.
It is best to put them in greenhouse in light but shaded to direct SUN area, make sure soil is moist but not too wet, over-wet soil kill young roots.
If no greenhouse, You should put your tree at light but shaded area without directly sun, and cover it with plastic doom/bag to reduce water loss.
When its tip has new leaves growing in about 2 weeks, it tell you that feeder roots to tip development are ready, When it has 2nd set of leaves, you should remove plastic doom and move them back to direct sunlight slowly.
Good luck