The leaf symptoms and pattern within the canopy are consistent with chemical injury including aerosolized or volatilized herbicide exposure. We've had windy conditions throughout the spring and continuing into the summer coupled with high temperatures. These are perfect conditions for causing herbicides to volatilize and drift to new locations. They commonly remain close to the ground producing damage in the lower portions of tree canopies.

The good news is that the vast majority of the leaves on your tree were not affected, so the tree has plenty of leaves to provide the carbohydrate resources necessary to maintain the overall health of your tree. The pattern and symptom progression indicate that it was a single-exposure event meaning that the number of leaves affected should not change.

However, while the symptoms are consistent with chemical injury, we must leave open the possibility that something else is causing the damage. For example, the symptoms could also be interpreted as "leaf scorch" caused by the inability of the tree to provide sufficient water to the leaf cells furthest from the roots. This could be caused by anything that reduces the flow of water up the tree from drought (lack of water in the soil), to root damage, to something plugging the vascular system such as a vascular wilt disease.

Thankfully, you sent a picture of the entire tree. The pattern doesn't "fit" with leaf scorch which tends to first appear high in the canopy rather than low in the canopy. On the other hand, if the symptoms continue to progress without additional exposures to an herbicide, you would need to expand your investigation including possibly sending samples of the affected leaves, including the petioles, as well as stems that have scorched leaves to our OSU Plant and Pest Diagnostic Clinic:   https://ppdc.osu.edu/