The sudden thick ice jacket it received made it too heavy to make it back to dispersal.
.... not too heavy - it was (apparently) degradation of the aerofoil section of the blade (NACA 0012 IIRC) which drastically reduced lift.
The mighty Wessex could (usually) just overtorque if overweight, but in this case I understand there was severe NR droop (which is very unusual in the Wessex .... unlike some other types....
), without excessive collective input and with both engines turning ..... which sounds kind of scary .....
But as a good friend of mine once said: "the older you get, the more clearly remember things that never happened......."