A former Swiss helicopter winch man friend of mine told me it was a practice rescue and was probably due to the pilot misinterpreting the rescuers hand movements, while trying to connect to the winch hook, as a signal to ascend.
Unfortunately the rescuer had not yet made the full attachment and it failed soon after their full weight had been taken.
The issue was apparently not un-common. Petzl have worked with heli teams throughout the Alps to developed some interesting devices making heli rescue much safer.