The dieback in the tips could be from a number of factors including lack of water, insects. or pathogens. You will need to look for other symptoms, such as cankers or signs of insects (including borrowing holes in leaves/bark) to narrow down the possibilities. It is possible this is simply due to lack of water. A likely suspect is Cypress Canker which causes tip dieback (https://pnwhandbooks.org/plantdisease/host-disease/cypress-cupressus-spp-canker). Cypress Canker symptoms often first appear at the top of the tree, which is what is happening with your trees. Furthermore, trees that are water stressed are more susceptible to Cypress Canker. Look for small black oblong fungal fruiting bodies and sunken cankers on the trunk and stems of the tree (http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/DISEASES/cankerdiseases.html). Prune out and destroy infected branches about 1-2 feet below the canker. You should be watering the trees regularly during the summer months or periods of drought to keep them healthy and less susceptible to insects and diseases. Other pests to consider include Phytopthora Root Rot, especially if you have waterlogged soils, or the Cypress Tip Moth (https://pnwhandbooks.org/insect/hort/landscape/hosts-pests-landscape-plants/arborvitae-thuja-cypress-tip-moth).