Sorry to read this late but it might have been that bag or a piece of kelp in the prop. You might consider tightening the steering tension so it's not deflected as easily because someone could get thrown overboard with a sudden swerve like that. My engine has done exactly what yours did, sometimes turning, sometimes kicking upwards. I never take my hand off the tiller at speed, only at idle due to safety concerns. Also, do you use the kill switch lanyard attaching it to your wrist or a belt loop? You should because if you go over and it's in gear and moving you may be cut by the prop or left stranded watching the boat keep going.