Hi Christopher, thanks for the question.

It is interesting that the other thujas are not showing similar symptoms. To determine what is causing the problem it would be good to determine what could be different about this specimen versus those close by.  Consider drainage, light (too much, too little), sprinkler pattern, recent construction, wind, etc.  Check the trunk at ground level to check for girdling roots.  Also, check to see if the tree is planted too deep (the root flare should show).  For spider mites, do a simple paper test (spider mite paper test), use magnifying glass to look for other insects.  For diseases, familiarize yourself with the different diseases and their signs and symptoms (Arborvitae Diseases (psu.edu)).  Look for anomalies to a normal leaf.

Check these things out.  Please let me know if you need additional assistance.