Does a Heat Pump run a defrost cycle when the AC is on?

Hello, First off, please forgive the ignorance. I'm just trying to learn the odds and ends of my heat pump (new home owner). I have 2 Bryant units and while they run fine it seems like maybe something isn't quite right when the thermostat "tells" the units to cut off. It makes a humming noise but the fan is not running. From the research I've done, this is normal for a defrost cycle. I was just curious IF and WHY heat pumps run a defrost while running the AC. I understand it while running the heat and it's cold outside...But running the AC when it's warm out?? please share your knowledge. I will note that it hums for a few minutes and then kicks right back on and cools. Rarely will it make the "gushing wind" sound that I've heard in some videos.