Same principle as a gyroscope: a turning wheel will tend to stay perpendicular or parallel to the direction of the gravity vector because if it starts tilting away from such orientation there’s a force that pushes it back.

Also works better with bigger wheels (if I remember it correctly the effect is related to spinning momentum).

I was pretty surprised when learning Physics and they show us how to derive the formula for that (which I totally forgot since that was over 3 decades ago).

Edit: Actually the gyroscopic effetc is just a part of it. See this article