why does it seem

They don't cause the car to flip over. The person driving the tow vehicle causes it to flip or is the catalyst that causes it. When towed responsibly, IE: correct vehicle, correct weight limits, correct speed, responsibly driven and no distractors such as being cut off; a towed apparatus on a trailer hitch is perfectly safe! The problems begin when you are towing outside the rcommended weight, have the wrong hitch for the weight, wrong vehicle for the weight, no trailer brake when one is recommended, going way too fast, failing to allow additional stopping distance, forced to panic stop or driving DUMB! It isn't the hitch at all; it's the operator 100% of the time.