Clay soil is usually what causes lilacs to die because people dig down into the ground.
With clay soil, it is best to mound upwards. That way, you don't create a "bucket effect" where water drains into the hole, but runs off. Clay soil is very wet, and stays wet for a long time. This is why you dig only a few inches down, and then mound up.
The symptoms for clay soil is usually smaller leaves, and when you dug up the dead plant, the roots looked like they were rotten or very small and broke off.
Hope this helps