It appears your lilac may be experiencing either a response to previous years of drought or a bacterial blight. Bacterial blight can occur when there is lack of circulation and moist conditions. I would recommend removal of the branch areas with the declining leaves. Also take care to rake up fallen leaves and twigs and dispose of in the trash.  In your photos it does look like the branches are tight and could benefit from pruning to allow air flow throughout the tree. I have put a link below to leaf spot disease and a video that instructs on pruning. With clean up and pruning your lilac will likely be healthier in the future. 

Leaf Spot Diseases
https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs#leaf-spots-and-shoot-blight-or-canker-1156510
How to Prune a Lilac
https://video.maine.edu/media/How+to+Prune+a+Lilac+Bush/1_1w8x1rwv