Wilko,
Disagree, soft ends most likely cause. Sitting at light with clutch pulled in will not increase likelihood of mushrooming like this. Greatest stress on ends is when clutch is activated, not when sitting there with the pressure plate (edit: and rod) at rest. Lack of lubrication at rod end may lead to overheating and, thus, annealing, (edit: ...though, this is the lever end.)
Richard L.