Some are more than 150 years old, and we hate to lose them.
Chances are it's probably the grass that's affecting the trees.
Native oaks are used to native rainfall amounts, and many varieties of fescue need plenty of rainfall to stay healthy. Many homeowners overwater their lawns, pouring as much as 100 inches of water on their turf grass in a single year. As a result, the oaks literally drown and begin to rot at the base. If you have an automatic sprinkler system, consider switching to manual mode and turning on the sprinklers only when absolutely necessary. It may be too late to save the trees that have already begun to rot, but it will give the unaffected ones a chance to survive.