Your heat pump had something wrong with it. Quite possibly it wasn't going into defrost mode or wasn't able to actually finish defrosting by the end of a cycle.

Your heat pump had something wrong with it. Quite possibly it wasn't going into defrost mode or wasn't able to actually finish defrosting by the end of a cycle. The way that's intended to work is that after running for a bit when the ice starts blocking air flow and insulating the coils on the outdoor unit it'll switch to cooling mode and turn on resistive heating. That quickly thaws out the coils and then it switches back to heating and turns off the resistive heating inside. I find it odd that you even had to manually turn on emergency heat to get any heating out of it. Normally if the thermostat is too far below the setpoint it'll automatically start using resistive heating as well (most thermostats will indicate "aux heat" when that happens). Maybe your thermostat didn't? Or it did and the reversing valve was stuck so it was just stuck in cool mode 100% of the time.