The symptoms you describe indicate that your trees are not able to pull enough water from the soil and there could be something affecting the roots (pests, disease) or the irrigation is inadequate.

How are you watering your trees? I mean how long do you let the drip system run, what size emitters do you have and how often do you water? It is essential that you deep water trees to thoroughly soak the root zone and then let the soil dry a bit before applying water again. With a drip sytem this usually means running it for several hours. The interval will depend on the season - once every two weeks in the summer, once a month in the winter.

Have you dug up the dead tree to inspect the roots? Perhaps something is munching on them. If these are Mexican Palo Verde trees (Parkinsonia achuleata) you may be dealing with the Palo Verde Beetle, who is especially fond of the Mexican PV roots.

Another thing to check is whether the trees were planted too deeply. This will cause plant death within a year or two. The area of the trunk where trunk tissue turns into roots must remain exposed to air. If it's covered up with soil the plant basically suffocates.

The fact that they are turning yellow makes me think that you are watering too much. Some trees might have better drainage than others and thats why some are doing better. I would think that a Palo Verde being native to the area would need little to no watering, except maybe an occasional deep soak in the Summer.

Check to see if they are being attacked by termites? Same thing happened to one in my yard. Died from the top down and limbs started falling. I noticed bugs and call the exterminator and he sprayed the tree but it was too late they had eaten too much.