When the trailer is unloaded, the tops of the tires lean slightly outward (toed-out, or duck-footed). When they are carrying the weight of whatever’s loaded, the axles straighten to a flat position and the tires come to a straight up-and-down position. When the load is too heavy, the axle bows downward in the middle, causing the tires to roll. You don't need H rated tires on the car but Ford put them on for liability reasons. The lesson here is to learn to check 3 things, 1) the tire pressure, 2) the lug nut tightness and 3) read the date code on the tire sidewall. Tires harden with age even if they aren't mounted on a car.