What could it be and how do I treat? They are very large trees
There are a couple of very serious diseases that cause leaf drop, so I advise seeking a diagnosis from a certified arborist.
https://www.gardeningknowhow.com/ornamental/trees/oak/oak-wilt-disease.htm
https://www.gardeningknowhow.com/ornamental/trees/tgen/bacterial-leaf-scorch-disease.htm
https://www.gardeningknowhow.com/ornamental/trees/oak/planting-pin-oak-trees.htm