Answer: Although they are evergreen, citrus trees do drop their oldest leaves and then grow new ones to replace them. What you may be seeing is this natural leaf drop. On the other hand, if it's a sudden occurrence after a long spell of good health, look for spider mites, whiteflies, or scale on the plant. Overwatering can cause leaf curl/drop as can excessive heat. If the tree looks otherwise healthy - new green growth and flowering on schedule, I'd chalk its behavior up to normal activity for the trees.
Best wishes with your citrus trees!
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