Lack of water!
The 25-30 ft tall oak tress leaves turning yellow in hot Texas heat. The mature trees should be able to weather out the drought, but it isn't always the case.
The leaves turning yellow quick! The lush green, become just greenish after 1 months of hot weather and no rain. The greenish leaves became yellow green in 2 months and start to brown out shortly after that.
Dumping 300-500 hundred gallons of water along the drip lines every other days for a week seems to have saved the trees.