There are many conditions that can cause a tree's leaves to drop off at the wrong time of year. Fungal and bacterial diseases, insect attack, and excessive moisture are just a few.
To find out what's going on, start by calling a Certified Arborist for a tree inspection. If the problem affects only the leaves, the arborist may recognize it immediately. If not, he may have to get a leaf sample sent to a laboratory for analysis. (Many county extension agents can help with this.) In some cases, leaf problems are a symptom that the entire tree is sick.