Never mind, figured it out. For anyone else who comes across this. The PSU has a 20 minute 'stay on' feature where if the fan spins up, it will stay on for 20 mins at minimum rpm before going back to zero. I guess this is done to prevent the fan from nudging around if the psu is sitting at the fan's on/off threshold. I think 20 mins is a little overkill but the fan isn't exactly loud so it works out.