Heat pumps behave like bidirectional refrigerators: they cool the building by heating the outside air or heat the building by cooling the outside air. In relatively mild, dry weather, this works perfectly.
Here in the Northeast US, it’s not such a bright idea. For about half the year, the ambient temperature is low enough and the humidity high enough that pumping heat out of the exchanger drops its temperature below the dew point, whereupon ambient moisture condenses on the fins and, given the temperature differential between ambient and coil, freezes solid.
In that situation, the efficiency of the heat exchanger drops well below zero: it turns on electric resistance heating bars to warm the inside air and runs a defrost cycle on the exterior heat exchanger.
Lather, rinse, repeat.
Perhaps the defrost cycle hadn’t started yet?