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Have the trees been getting the usual amount of water? Has it rained more or less than usual this summer? Does your brother's yard have a layer of hardpan or clay? You do ask if poor drainage could be a factor. Does his yard have poor drainage? Are other plants in the yard showing any stress (yellow or burnt edges on leaves)? Is there white residue on the surface of the soil? Does your brother fertilize?
Palms are very sensitive to salt buildup in the soil. If there is a hardpan or a clay shelf that's causing poor drainage, salt could be building up in the soil due to inadequate flushing. The palms would be the first to be affected.
