You forced it to make a bunch of new replacement growth in summer which proved to be susceptible to mildew because of the timing. (Lilacs can mildew anyway but no doubt you are seeing it to this extent on this specimen because of the pruning).

It's powdery mildew. Extremely common with lilacs, especially late in the season. Unsightly but generally not particularly harmful.......more cosmetic than anything else. Clean up all the leaves thoroughly when they fall and destroy.

Thank you embothrium and gardenGal.