Not only did my lilacs start to drop their leaves right after they flowered but the apple trees are dropping their leaves early. A large elderberry is almost totally leafless. While this could be some sort of fungus or bacterial disease my guess is it is stress. There are too many different varieties acting in exactly the same way for it to be a single disease. We have had very high temperatures with torrential rains. Excess heat slows photosynthesis. The trees are so stressed out that they can't support leaves. The apple trees are holding on to their fruit but the fruit is small. The trees are just shutting down to preserve the integrity of the main body. It's sort of like hibernation. Some woody plants are just less susceptible to climactic conditions or are growing in an area that protects them from excesses. My white lilacs are fine but they are very old, very large and are somewhat shaded by the house.