The tree must be under some kind of stress. Has there been any construction or digging in the root zone? Could it have been hit by herbicide drift? Is the trunk damaged by lightening/animals/mower bruising? Has the soil level been changed under the canopy? Any of these environmental stressors could cause your tree to drop its leaves. The best thing to do is keep an eye on it and see if it is able to releaf on the bare branches. Don't do any pruning; if the tree does releaf, it will need energy from all of the leafy canopy to recover.