Spider mites will discolor Arborvitae, but I've not observed them to kill the plants. Lack of water in summer will kill the plants, especially since the hot summers of 2015. This year of course was particularly hot and dry. But you indicate this is not the problem, so that leaves the other cause of total dieback of these plants, which is Phytophthora Root Rot, which usually occurs in plants in poorly drained situations. Without more information and some photos, or a site visit, I can't really tell you much more than that. If you would like, please contact me by email directly at the address below. Thank You!