See at how flat the angle of the ladder is? They put down struts before extending the ladder - otherwise it would just tip over, especially with the second guy getting on it - once it’s extended like that, there is no moving the truck.
edit: see the first fireman waving frantically at the second one? He doesn’t want the truck to back up, he wants the second guy to get down so the ladder doesn’t tip as it is extended further from the vertical. The horn might actually be a load/balance warning. That’s why the ladder movement is interrupted - the control unit recognizes it is near the tipping point and refuses to extend further. As the second fireman climbs down, the load decreases and the ladder can be extended further.
