Your stand is probably of lower quality. Vibration caused by lower notes and forceful playing contribute to moving the stand, and it also gets worse with time by loosening the joints, screws and bolts (natural effect).
Another thing that can contribute to such problem is the levelling of the floor over which your stand lies. Joints between tiles, poorly settled wood floors or subtle inclination can lead to unpleasant unstability of the stand.
One possible solution is to put some form of wedge beneath one or two of the corners of the stand. Press down either side of the stand and check if the corners move up, leaving a gap to the floor. Put the wedge there, adjusting its thickness until the stand settles firmly.
Another good measure is to periodically re-screw every joint and connection of your base, checking for unreversible damage to the screw holes. In such cases, you can exchange current screws for longer or thicker ones, always taking care to avoid cracking/breaking the stand material due to overpressure.