If you mean on a commercial airliner while parked at the gate, this is due to the control surfaces being operated by hydraulic cylinders (versus cables or linkages, like you might see in small general aviation airplanes). When powered down, the cylinders lose pressure and the rudder will tip to one side or the other depending on a gust or gravity or whatever other force may act upon it. This corrects itself once the engines power up and the hydraulic accumulators recover.
If you mean while in flight, this is rudder trim. On large airplanes, this is mostly controlled by the Yaw Damper, which automatically manages the yaw of the airplane by manipulating the rudder. On small airplanes this is done via a trim tab which is either bent to the correct angle or maybe be cockpit adjustable (like the one pictured below). This small tab causes the whole rudder to deflect the opposite direction, which in turn stabilizes and “coordinates” the airplane in level flight. During turns, the pilot would use the rudder pedals to “coordinate” (or account for adverse yaw). A rudder is rarely ever simply riding along in it’s vertical state.
Source: Quora
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