If it is only oldest leaves (lower on the stems) That is likely to be natural aging. Individual leaves only live for a few years then die.
Old leaves dying can also indicate nutrient deficiency. Plants survive starvation by removing goodness from older leaves and transferring it to newer growth. - what fertiliser have you used and how often?
Dropping leaves can also be a sign of dehydration. More likely where people are actively trying to avoid overwatering IMHO. Trees kill off some leaves to reduce water needs and to preserve fresher, young growth. If you suspect underwatering simply increase water frequency. Change of season also contributes to likely underwatering. New season growth demands more water while we are still watering for winter = dehydration. As the pot gradually fills with roots there's also less space for water in the soil. That means the same watering as last year may not be enough this year. In this case, repot to re-establish air and water space.
You seem confident the soil dries out between watering so overwatering is unlikely. If in doubt, you should tip the root ball out of the pot and check soil moisture and root growth.
