These plants need full sun to begin with and rich well drained soil. Something tells me that it is getting neither and is giving up.

has it been heat/dry stressed? if yes it could be going into autumn leaf/leaf drop mode.

I'm told they do best in a slightly alkaline soil so a dressing of lime might help if you think that could be a reason. Also they don't like to be crowded in ,which is what happened to mine, hence it's ugly shape, which Bamboo has just advised me on. (See my post)