Based on your information, my belief is that your tree has wooly aphids, the ants are collecting the honeydew these insects drop. The honeydew will also draw the hornets. Many woolly aphids can suck enough sap out of a branch that the leaves at the end of the branch begin to yellow and die because they are not getting enough nutrients and water.
Use insecticidal soap to knock them down in numbers. Also it may be to late to save the yellowing leaves.