Why do planetary all rings seem to be on the equatorial plane?

When i first saw a picture of Uranus and it's rings i was surprised by their location, 90 degrees from the axis of rotation.

This led me to believe that the drag of the planet on spacetime created a gravity well that the rings 'fell' into. Recent studies into frame-dragging seem to validate this idea.

Are there any other studies in this area i can look into?