Answer: What you describe is not normal so I would do a little more investigating. Your plants may have a spider mite infestation. Check the center of the plant for tiny webs. Spider mites are tiny and difficult to see without a magnifying glass, but they usually leave behind some webbing. You might also try holding a piece of white paper in the center of the plant and tapping the main stem sharply. Spider mites (and other debris) will fall onto the paper. If you find ground pepper sized black dots and the paper - and if they move - the plant has a spider mite population. Control them and no more browning should occur.
Best wishes with your arborvitaes!
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