This leaf cupping and other distortions are typical symptoms of exposure to phenoxy herbicides.   Lilacs are pretty sensitive to the phenoxy herbicide called 2,4-D  (2,4-dichlorophenoxyacetic acid).  It is a component of many herbicide products that are applied to lawns to kill lawn weeds like dandelions. 
If applied on hot days, 2,4-D may volatilize and drift several hundred yards. Similarly, it may be moved quite a distance if applied when it is windy.   Very small amounts of 2,4-D that become airborne can injure sensitive plants like lilac, redbud, aspen, catalpa (photo), hackberry, grape, tomato and others.  Check other plants in your yard and neighbor's yards to see if any exhibit similar leaf cupping or other distortions.
This "herbicide drift" that causes injury to non-target plants is pretty common, despite herbicide product label warnings not to apply when it is hot or windy.
Usually, the small amount of 2,4-D that causes leaf distortions on sensitive plants is not enough to kill the plant.