If it's a Powergear slide with rack/pinion, the free-wheeling brake on the actuation motor has likely been released. There should be a lever on the motor that is fairly easy to flip back and forth that releases the motor to free-wheel to allow for adjustment. It happened to mine, and although I keep the slide locks in place when going down the road, I noticed in the mirror that the bottom of the slide was slightly out. Took me some thinking on it, but I discovered the motor brake lever, flipped it back and all was good.