Answer: Environmental stresses can cause cupping of the leaves as can exposure to the broadleaf weed killer 2,4-D. If you used a broadleaf weed killer near the lilac the chemical can translocate into the soil and reach the roots. Overly wet weather can stress the roots, and cause the same symptoms. I'd take a wait and watch position. If future leaves grow normally, the problem was weather related. If they show the same symptoms, I would suspect herbicide damage or even the very early signs of lilac leaf blight (a fungal disease).
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