Experts and BHG readers answer.
Deformed leaves on lilacs
Are you using weed killer on your lawn? Your description of curled leaf edges and toughened leaf margins is a typical symptom of broadleaf herbicide injury. The herbicide 2,4-D can volatilize and move through the air fairly long distances. Dicamba can move through the soil. (And lilac roots can spread 3-4 times the width of the branch spread.) Either mode of action could result in the symptoms that you've seen.