It is lokely simply lack of enough moisture. It has been a very dry summer and even arbs might need extra water. Set a hose trickling at the base of each one for a couple of hours about once a week or so to help them catch up. Fall watering of arbs and other evergreens will be important to hydrate them before winter especially of the weather continues to be dry.
Less likely would be overspray or other contact them might have has with weed killer spray that was applied to the lawn nearby.