Answer: Your tree sounds like it it stressed in some way. It might be suffering from verticillium infection for example. There is a description of this at the following page (you may need to cut and paste the complete url into your browser to make it work correctly)
I would suggest you work with your county extension or a professionally trained and certified nurseryman or a professionally trained and certified arborist to try to determine specifically what is causing the defoliation. Once you know that, you can decide how to proceed. I'm sorry you are having trouble with your tree.
Q&A Library Searching Tips