Could very well be lack of water. Stressed arbs are often attacked by spider mites. I recommmend you have a certified arborist - a trained tree doctor do an onsite inspection (you can find one here: http://www.isa-arbor.com/faca/findArborist.aspx). If you wait until next season and you may have to replace them. Best and Happy Yardening, Nancy.
Arborvitae Shrubs Have Brown Areas
Question From: L. Andresen - NEW YORK
Q: I noticed for the first time that many of my Arborvite shrubs have brown areas on them. (this was noticed in late August) This has never happened before and I am usually on top of them. We sprayed them for insects 2 weeks ago. Should I spray again? I have no idea what the problem is and am concerned. What should I do now and will it return next spring (brown areas)