The fact that only one of your oak trees is dropping leaves may mean that the soil around it has been compacted by heavy foot traffic or that soil structure around it is simply different from that under the other tree. If the tree is on a slope water retention may not be adequate.
I believe you will be able to find some indications of what is causing the leaf drop in an article from Texas A&M. The article describes symptoms of oak wilt and of other less serious diseases of live oak. These can often be diagnosed by examining the fallen leaves. A pdf copy of one of the accounts can be downloaded here. These bacterial and fungal diseases are most commonly seen in seasons of good or heavy rainfall rather than drought.
If the leaves do not show indications of disease, I would recommend that you pay attention to the ground area surrounding the tree in question. If the soil dries three to four inches deep, it’s time to water. A sprinkler system generally does not provide enough water for trees. Give the tree regular deep watering by leaving a garden hose trickling on two or three areas near the drip line for several hours. This method is better than watering near the tree's base.
If it appears that you may have an oak wilt infection you should contact the Texas A&M office noted in the above web site right away. Other types of leaf disease are usually not particularly serious and may not harm the tree. New leaves will not form until next spring.