The brown spots that appear on arborvitae in late winter are likely from winter injury. Especially since yours were just planted last year, they won't have an extensive root system and winter drying/dessication is likely. If the brown areas are small, you may want to wait and see whether the buds on these sections were also damaged. Wait until new growth starts on the tree this spring. If now new growth is seen in the brown spots, trim them out. The healthy areas of the tree should eventually fill in the brown areas. Next fall, make certain that the trees are watered well going into winter.