Sprinklers are going in the pic. Far fence is wet. Its possible that the roots of the trees have reached under the sod and pulled out the moisture during a recent hot spell. Check for roots. Check for full sprinkler coverage. However I think I see an area along the fence in the background where the strip of grass is dead, yet there is a fence and no trees. Something is odd about the pattern. In most places there is a healthy green strip of grass nearest the garden area.
Suppose a powerful grass killer was used. Or --a powerful grass killer was used in the soil of the potted plants ...used before the potted plants were purchased.
I suspect the customer applied some sort of fertilizer or chemical to control weeds or kill insects on the soil along the trees. Do the trees look healthy? Did he drain pool water here after cleaning the filter or using a powerful algicide?
Or was the area reseeded after installing the trees (reseeded with mostly rye--which died out in the summer's heat). The green spots may be zoysia and the brown, some other kind of grass.