As the roots go, so goes the top. That's why they suggested a drainage problem. However, if the drainage has not changed in the latter part of the last 15 years, then implication is it is something else.

I do not know if the drainage has changed since they were planted. We have only been here for the past 2 years. There are photinia's growing along the same fence and they do not have the same problem. If it is a drainage problem, what can be done to fix it? We have clay soil. Will amending the clay soil around them help? Thanks.

Those need good drainage, would not be likely to hold up better in a damp situation than the laurels. Probably you have a pathogen at work, perhaps honey fungus (Armillaria). The way it appears to be moving down the row would fit something like that.

See about getting diagnostic assistance from California Cooperative Extension. They have had a large presence on the internet in the past and will also be present in telephone directories.

Substantial amending of planting hole backfill in damp soils is liable to produce sump-like conditions, make the soil around newly planted specimens wetter rather than drier. Digging in amendments around existing shrubs would be expected to result in damaged feeder roots without adequate improvement in drainage.