As your heat pump was not keeping the right temperature, the control board was not giving the right orders to heat and defrost the unit. Usually the unit switches into the air conditioning mode and turns on the electric heaters. If the electric heaters don't go on, or there is not enough electric heat, the temperature will drop indoors and there might not be enough heat to defrost the unit.
As there is no way to know what the cause was, you might have the luck to have it actually occur when a repairperson is there. Most likely it will occur at a very cold night when most folks are asleep.