From the border agent's perspective anyone can be a threat. You should approach these situations avoiding any actions that may confuse you with a lunatic or criminal. Not sure if that was the case here or not. Hopefully it was on camera so we can know the true story.
So the correct response is to supplicate before the guard, avoiding any visible signs of irritation, hurry, etc. It is generally preferable to put on the most benign facial expression one can muster to avoid threatening or annoying the guard, who has the power to waste several hours of your life on a whim, and can cause far more harm if he chooses.
