Are all the leaves falling off or is there a pattern to which ones are yellowing and falling off? Is it primarily the older leaves? Ones on the interior of the plant?
It could be a disease problem, such as powdery mildew or other fungus, although usually you would see other signs of the fungus on the foliage. Another possibility is some sort of problem in the root system. Either overwatering or hot, dry conditions could cause yellowing.