Ladders has been added several versions ago for those airplanes where the door's height is lower than the lowest height reachable by the stairs vehicle. The ladder will come ONLY if the door's height is higher than 1.85 mt. but lower than 2.20 mt. so, the reason why you are seeing it, it's the height of the doors on that airplane being in that range. In previous versions you wouldn't see anything on the same airplane.
But since you are not saying which airplane you are using and/or if you configured it, it's difficult to say if the problem is due because you are trying to use an unsupported airplane without having configured first, or you made some kind of mistake editing the doors on a supported airplane.
