Veins yellow symmetrically would point to lack of Nitrogen. All yellow except veins dark green first in young leaves: lack of iron, zinc or mg. Asymmetric patches of yellow in leaves and corrugated leaves would point to Citrus greening disease.
As this is seems to appear in old leaves I would assume lack of Nitrogen. The tree is either creating new roots and needs nitrogen or their old roots were damaged by stagnant water and is getting rid of some old leaves to compensate it.