Your plant is susceptible to loss of buds in a late freeze but should be a good plant in your area over all. Possible problems may have come from leaching of salts used on a nearby path or driveway into the lilac's soil or soil that is not draining adequately. I cannot see how high the pebble mulch is coming up the trunk but it should be moved away from the trunk to avoid excess moisture that can lead to rot and bacterial disease .