In April, I planted four Dwarf arborvitae little giant. Three are doing beautifully. One is getting brown leaves and when you touch them they fall off easily. We have not had a drought and have had ample rain. I don't know what to do with this tree. Can you help.

if these are interior to the plant, this can be normal as each year they will lose some needles. You may be losing more than normal due to transplant recovery on that specific plant due to variables like soil, exposure, root system development. If exterior needles, be sure that the root ball is actually moist itself. There are no substantial roots in the back fill material yet. There also could be a spider mite population building up and causing damage. Suggest contacting your local extension office and getting a more local and focused reply to your concerns