A rig was being prepared for loading out and transporting from a third-party rig up and commissioning site. The task at hand was to lower the main sub base section (weight 12 ton) of a 750 hp carrier-mounted rig. A third-party crane and operator was being used to rig down the main components. The onsite Rig Manager was directing the loading process at the time of the incident. A 25-ton crane had been engaged to lower the main sub base and rig floor section to the ground level. The load needed to be lifted and moved to the right for storage of the sub base. The crane operator had jibbed out all stages of the boom prior to taking control of the load. During this operation, the crane operator was using his cell phone. One of the rig crewmen was on the rig floor section knocking out the pins from the stabilizer beams. It would appear that there was slack in the lifting sling and following the removal of the last pin the full weight of the sub base was suddenly taken up by the 25-ton crane. The last section of the extended boom failed and buckled under the weight. This caused the sub base / floor section to free-fall to the ground approximately 12-14 ft. The employee on the rig floor section who was knocking out the pins managed to “ride” the load to the ground and escaped any injury. Damage to the sub base structure consisted of bending and stress to the main six stabilizer beams. The crane was left in position following this incident and all personnel cleared of immediate area. The area was barricaded off while inspections and investigations into this incident took place.