Arbs will lose some interior needles every fall and is normal. Over watering water logs the soil and roots do not get the oxygen they need and cannot move water up into the needles. Letting the soil dry out reintroduces oxygen into the soil profile. As they are new, you may lose more needles than normal. With a limited root system, consider a temporary wind break for the winter to reduce water loss from the needles. Water as needed next summer as they are not yet established.