They have hollow stems and there may be some sort of perforation or hole in one of the branches that has allowed an infection in.

You may have to renovate the whole tree by hard pruning into heavier wood. Seems drastic but the resulting new growth looks amazing with massive leaves, much bigger than normal new growth off one year wood. It should recover well if you do this next spring just before new growth.

Don't leave the good stems on either or you will end up with an unbalanced tree, do the whole thing, you will be very surprised at how quickly it grows back.