Everytime you see one of these operations, you are watching a dance between gravity and ‘critical angle’ (CA) and ‘center of gravity’(CG). If the crane is close to vertical and within tolerance for its load, there is no problem unless an outside force intervenes; high wind or support shift. Move the crane arm & load away from vertical and you move the CG and you start playing the dance music. The critical factors are the weight of the load and the angle of the crane. The higher the angle, the higher the weight capacity and vice-versa. The CA is where the arm & weight & extension exceed the base weight and put the CG outside tolerance.
When the crane operator feels that little shift, he/she needs new underware.
I was operating a crane while setting off a large electric motor inside a plant. I had the long part of the truck facing toward the boom and motor. The other guys grabbed the motor to guide it in and I noticed that the cab was tipping forward. Extended too far. I let out some cable and the crane’s rear outriggers set back down on the ground.
Drank a lot of beer that night.