1993 Chevrolet Silverado dash gauges

That's how they normally work. They aren't the normal extremely reliable gauges with heating elements that Ford and Chrysler used for decades, and they aren't the normal "weak magnet / strong magnet" gauges GM used without problems for decades. They are now computer-controlled because, ... Well, ... No one knows why the insane engineers feel the need to add a computer to every conceivable part of our vehicles that never needed a computer before, but that's what we have. Your gauges' pointers are placed to the desired position by a "stepper" motor that is positioned by the computer in the instrument cluster. There is nothing spring-loaded and nothing to return the pointers to "0" except the computer. When you turn off the ignition switch, the computer shuts down and the pointers will stay wherever they are. Occasionally one will move around a little on its own due to gravity, but that's just a coincidence.