The following sites should help you determine what is causing the lilac leaves to yellow and die. Examine the lilacs after you have read this material.

https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/lilac/

https://extension.umn.edu/yard-and-garden-news/lilac-issues-season

https://extension.umn.edu/trees-and-shrubs/lilacs#common-problems-1923161

https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs

https://hort.extension.wisc.edu/articles/bacterial-blight/
A cool, wet, rainy, spring season favors development of lilac blight, especially if rains follow a late frost or winter injury. "lilac bacterial blight," this disease is caused by a bacterium Pseudomonas syringae pv. Syringae. If your lilac bush has this infection, prune and burn all infected parts as soon as you notice them. A spray of copper sulfate during the early spring each year should help prevent the problem before the buds begin to break. Preventative measures include removal and destroying of leaf debris and pruning out dead branches.
Prune infected branches, cutting well below the diseased tissue. Disinfest the pruning shears between cuts. Apply a bactericide to protect healthy shoots.

If the fungal disease Lilac Pseudocercospora leaf spot is present the symptoms are yellowing then browning leaves, dieback.The PDC emphasizes sanitation as it seems that the spores of Pseudocercosporacan persist for several years on plant debris (leaves, stems, dead flowers, bark). Clean up fallen leaves to help reduce the likelihood of reinfection next season. Renewal pruning allows more light throughout an older plant and encourages new stems to grow and flower. Use a lopper or hand saw to remove a third of the thickest stems at the base. Do this every year until all large stems are removed.

https://extension.umn.edu/yard-and-garden-news/lilac-issues-season

https://extension.umn.edu/trees-and-shrubs/lilacs#common-problems-1923161

https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs

https://hort.extension.wisc.edu/articles/bacterial-blight/