A lot of times, a car will lean if the screws holding the trucks in place are too loose. I had a number of Athearn 34' hoppers with this problem, very exciting when you run trains with loose loads. Those loads may have contributed to the problem too, as a loaded hopper weighs about eight ounces. The problem was traced to the screw mounting bosses, which protrude from the underside of the slopesheets. Most of them had split, rendering them useless for holding the screw in place. Most were fixed by glueing the bosses back together, allowing them to cure fully before reinserting the screws. Others required glueing, plus filling the hole with styrene rod, then redrilling for the screw.
In this photo, the Athearns are empty, while the loaded car is from Stewart.
Wayne