Not 100% sure if this was the case, however it seems like a possibility
CSFF (Cold Soaked Fuel Frost) / NEI (Non Environmental Icing).
I would imagine the 757 had just arrived from the US. Spending a long period of time in the cruise ensures that the temperature of the fuel in the main tanks (wings) gets pretty cold. I am basing by answer with knowledge of the 737NG, however I am pretty sure the 757 will also draw fuel from the centre tank before the main tanks. This leaves the fuel in the wings untouched for a reasonably long period of time. As the aircraft is in the cruise the outside skin gets extremely cold, so naturally the fuel in contact with the wings will also get cold. As the aircraft descends into the warmer air, condensation may form on the aircraft's skin. Alternatively moisture may be picked up as it descends through cloud layers etc. Remembering that the fuel in the wings is still cold, the moisture on the aircraft skin, most particularly the wings, can freeze resulting in a layer of ice forming on the upper and lower services. In most cases that I have experienced the ice usually forms on the lower surface as the fuel that would normally have been in contact with the upper surface of the wing has been used. Often deicing isn't required as when you refuel, the warmer fuel temperature in the tanker melts the ice. However the day that you describe sounds particularly cold and I'm guessing it was probably fairly early in the morning (when surface temperatures are still on the low side).
That's my best reason for such deicing activity. Considering the day in question (crisp, clear and dry with no signs of snow, fog etc)