Could be old age, could be ground based nasty of some sort.

I have lilacs here that are older and they are getting mushrooms growing on the base and dying off.

I cut one down and the middle of the trunk was rotted.
There may be treatments for it if you check with an arborist.

My lilacs act this way if they have scale. I use a systemic product and it works well on everything except euyonomous. I just can't win the fight when scale finds them. The pink lilacs seem to need extra water in my yard or they wilt and start to die, but I don't think that is your problem.
Mybe find more detailed infmation at a lilac sellers website.
Take some kind of action now, or you might lose the whole shrub.