Have you been good about fertilizing regularly? Low fertility with regard to one or more of nitrogen, phosphorous, potassium, or magnesium can cause loss of older foliage as the plant robs those nutrients from older leaves (to provide the building blocks for new growth) before they're shed. About every 3rd or 4th watering is good for summer. For winter, every 4th or 5th watering is about right. If you get a good rain during the interval between fertilizing, it should be counted as a watering.

Not sure if you mentioned when the plant was last repotted, but root congestion also causes loss of old foliage, and as the foliage loss progresses, so much foliage can be shed that all remaining will be concentrated at branch tips in the immediate vicinity of the branch apex/ apical meristem where extension growth occurs.

Al