I wonder, also, if they are getting too much tlc
I actually do not mulch my agapanthus (they are in bare soil, too) nor have I ever fertilized them. I shouldn't admit it but they are by far the most neglected plants in my yard and they are thriving - almost too much so as they need constant dividing. My agapanthus get watered 2 or 3 times a week typically but never heavily in any one sitting (they get the over-spray from the sprinklers only).
The ones that I mentioned above that are currently in shock are those that I moved into an area that has rich soil and is more heavily irrigated. I am keeping an eye on them and I may have to move them to the outskirts if they don't improve by autumn.
It is odd that this is the first year you have had an issue with your agapanthus, though. Has anything changed in your garden? Could you possibly have moles or gophers? Usually by year 6 or 7, these guys should be like cast iron.
By the way,
to ATP!