I'm in Ho Co too. It's the drought. Trees will cast off a lot of their leaves in response to drought, starting with the shade leaves on the inside and bottom of the tree. But unless they are newly planted, they will be fine. There is no drought that would kill a healthy oak tree.

It is probably just the extra dry July and early August. But my neighbor lost a huge, formerly healthy oak tree.

There is a disease called sudden oak death that may have come to the east coast from California recently with nursery grown rhododendrons acting as carriers. Though, after some research we decided that what was killing her oak was a disease called oak decline which has been in this area since before 1900. Drought makes trees more susceptible.

An internet search should turn up pictures and lists of symptoms of both diseases. All the effort and money she spent trying to save it came to nothing and it had to be taken down because of safety issues.