if the unit is runs aux in two instances : the heat pump can't keep up and the heat strips are being used as 2nd stage heat, or aux heat is being called for by the defrost relay sending a signal from the outdoor unit. the latter happens when the system goes into defrost. does not sound like it is in defrost because the line is hot. it's normal for the hot gas to be routed to the indoor coil during normal heat pump operation. if aux is showing up at the beginning of the cycle, this could indicate a false signal from outsid, possibly a defective defrost board. verify operation outside at the beginning of the heat cycle (cold air discharging) and check aux signal from defrost relay. should not be present.