Lilac shrubs have been challenged this year. The last few years of drought, followed by our large volume of rain, has resulted in blight for many of the shrubs. If your shrub has blight clean up of the leaves and branches is important. Here is a link to information regarding blight. https://pnwhandbooks.org/plantdisease/host-disease/lilac-syringa-spp-bacterial-blights Your lilac shrub appears to have a greater amount of decline. This could indicate insect damage from a lilac borer. If this is the case you will notice borer holes on the trunk. If these are evident the borer will also be present within some branches. Below is a link with additional information regarding lilac borer. 
https://www.uidaho.edu/extension/ipm/garden-pests/arthropods/lilac-ash-borer