It's hard to tell with this extent of browning, but over-watering is indeed a possibility as Arborvitae dislike both excessively wet (or poorly-drained) and excessively dry conditions. Whether or not they were planted as burlapped specimens that didn't have the root ball covering removed upon planting, or potted plants that didn't have any matted roots loosened, could also impact how well they establish and how vulnerable they are to soil moisture levels years after planting. There is no cure if the roots have developed a root rot infection, and since Arborvitae have limited capacity for regrowth from older branch tips, it remains to be seen if the plants are capable of recovery. (We suspect not.) The brown tips, at least, will not recover and would need to be pruned off, though carefully so as not to remove much of the healthy green foliage behind them.

Miri