I moved into a house with well established arborvitae. They are probably 15-20 feet tall and create a large privacy hedge. I noticed today that some was turning a light shade of green and it seemed like it was slowly dying. The foliate shoots are brown and the branches seem to be grey. This is pretty consistent across the 40 foot hedge.

All needle evergreens will drop the oldest set of needles at some point during the summer. these are the needles to the interior of the tree. Depending on recent rates of growth i any year, visually this can appear to be nearly 50% of the needles on the tree. Since it is occurring nearly uniformly, this fits. Diseases do not infect in this manner. For all the trees to be impacted, the only other factor would be something environmental like air pollution. If you look through your neighborhood, I suspect you will see similar conditions on other arborvitae.