I have seen that too with my Adeniums, it will slowly dry out the leaf tips and drop the older leaves especially if those leaves are the leftover during dormancy or previous growing season, and the new ones will sprout later on. I used to wonder if my plant is sick and observing other plants here, they do the same, drop the older ones, and come up with the new one. Just natural leaf senescense as seasons change, that is how I see it. The plant is slowly reallocating its resources to new growth.
I would be more concerned if the branches, stem or caudex is getting soft and drying up..then a disease is on hand.
