As liquid mass turns into ice, it releases heat. Basically think of it as a solid changing to a liquid, the energy "trasnfer" has to give off some sort of warmth. Anything in motion will give off some form of heat.
As the liquid mass freezes on contact, the energy expunged creates a tiny little pocket of heat. As the rates become heavier, more heat is released and eventually the ground level comes above freezing.
Lighter ZR is letting less heat off as fast, because the droplets are smaller or the rate of precipitation as a whole, is lighter. From what I have seen, the bigger ice storms have occurred in heavy precipitation scenarios, when the surface was too cold (26-28F) to be overcome by the liquid to solid change, and the end result being destructive ice accretion.
At least this process is what I was taught in my reading.