Another possible reason could be honey fungus infection. if you go to the base of the plant and lift a piece of bark there may be a white mycelium sheet underneath this will also be under the bark of some of the roots. also the wood of the roots will be soft and wet. If you confirm honey fungus remove the dead plant and as much of the old roots as you can.
have you had any other plants die in the past or in next door garden?
There is no chemical cure or control for honey fungus. check the RHS web site for more detailed information.