Gravity doesn't pull you forward, it pulls you down. Unless a particular human never learned to walk or sit upright, their natural tendency in balance is to hold themselves in-line with gravity*, not necessarily at a perpendicular angle to the surface they're standing on. (There is some input from the eyes, which trick house attractions and the like exploit, but we're usually pretty good about it.)

This is what learning balance is all about, and most of us pick it up by the time we're two years old. Segways detect the human riders' effort at balancing (which most do without effort or awareness), and exploit it.