It's very hard to say exactly what has caused the leaves to drop from the tree. Some types of avocado naturally shed their leaves prior to flowering but there are also a number of diseases and pests which could have caused this but avocados are also sensitive to any type of change in their environment which can cause them to shed their leaves. Did anything change suddenly? How hot is is getting where you are and how long has it been at that temperature? Can you see any signs of fruiting? Any signs of a pest such as mites? Please also feel free to upload some images of the tree and leaves as they could be helpful in figuring out the problem.