Discoloration on Arbor Vitae Trees
Your arbor vitae is showing normal autumn needle browning. Although we think of evergreens as always being green, they actually drop old foliage in the fall just like deciduous trees. However, they only drop needles that are about 3 years old, so you see browning of needles in the interior of the shrub or tree while the exterior, newer needles remain green. Some autumns this process is more noticeable to homeowners than other years, because the year of needles that is browing was a good growing year for the tree and it put on a large volumn of new growth that will now drop. When the needles drop, it will look perfectly fine.
The dropped needles will eventually form a little mulch of old needles at the base of the tree, which will decompose and feed the tree and help keep down weed competition for the tree.