These plants are so very valuable. There are a number of things that could be in play, definitely not one or two. If you are in the Bay area you just have to contact your nearest Cooperative Extension Service and get an arborist or Master Gardener out to inspect your hedge. To do this over our site is helpful but you do not want to lose these plants and you really need and to LOOK and TOUCH the plant and check your soil, your watering program, like now. Very inexpensive. What is the closest University?
You'll need to send us more pictures, more information you might not have at hand...close ups of the leaves, magnifying glass to look for insects, soil analysis.
I'd like to know your water pressure, how long the pop ups water these trees and whether these plants have had any fertilizer, what that was and how much and how long ago. They should be watered deeply and then allowed to dry before any more water is added.
Has anyone pruned? Always make sure the bottoms are wider than the above foliage and that the tools are sterilized with alcohol first!
Are you able to talk to the former owners about their maintenance practices?