Answer: If there's no root rot, I'd suspect spidermites. These tiny little pests like dry conditions such as you'd find in the center of an arborvitae. Check for the pests by holding a piece of white paper under the branches towards the center of the plant and shaking a branch or two. Some debris should fall onto the paper. Spider mites are about the size of a pepper flake but they move. If you see small black specks and they move, your arborvitaes have a population of spider mites. Sadly, once the foliage on your arborvitae dies back, it won't be replaced. Your plants may continue to put out new top growth but it is unlikely that they will fill out again. I'd be inclined to replace the plants. Then, to protect them from a spider mite infestation, hose them off periodically during the summer months.
Best wishes with your landscape!
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