Do you see any caterpillars on or under the leaves? The spiny elm caterpillar could be the culprit. They are about 2 inches long, a black body covered with tiny white dots. There are other species of caterpillars, like forest tent caterpillars also. They are rarely in high enough numbers to injure otherwise healthy trees. You could use a pesticide spray if the numbers are high enough to warrant. If you don't see any caterpillars now, the damage may already be done and you'll have to wait to see how the tree survives.
If you have additional questions, let me know.