Before uncoupling a trailer, why is it important to ensure the trailer is supported and on solid, level ground?

Before UNCOUPLING, ensure the trailer is properly SUPPORTED and on SOLID, LEVEL ground so it doesn't FALL, ROLL, or SINK when the tractor pulls out. CDL combination. KEY STEPS: park on solid, level ground (landing gear can SINK into soft ground, dropping the trailer); lower the LANDING GEAR until it firmly supports the trailer (crank to lift some weight off the fifth wheel); set the TRAILER PARKING BRAKES and/or chock the wheels (prevents rolling); only then disconnect lines and unlock the fifth wheel; pull the tractor out slowly. If the ground is soft or the trailer isn't fully supported, it can drop or tip when the tractor leaves — damaging equipment and causing injury. Knowing the importance of solid ground and proper support when uncoupling is commonly tested.