What makes a bike tilt one way or the other when riding hands free?

Obviously bikes can ride very straight and stable hands free at the right speed, and rake or trail play a big part in stability etc, but when a bike does want to tilt one way or the other, what is the deciding factor? Asymmetrical weight from the drive train? An asymmetrical frame? My asymmetrical body? Wheel dishing?? Out of true wheels? Tiny differences in hub alignment?? What if we assume the frame is perfect and the wheels are true?