Rig crews utilizing trucks and a crane were in the process of rigging up. The pony subs were set and the top subs stacked. The crane set the first substructure spreader in place, however, the spreader did not set down into the substructure pin ears all the way. At this time the crane line was unhooked from the spreader. This action resulted in an overhead load (i.e., the spreader) not being secured. It is not clear who instructed the crane swamper to disconnect the crane line from the spreader. A gin truck was hooked on to the driller’s side substructure at the bottom and onto one side of the load hitch. When the gin truck lifted the substructure it spread the substructure more than expected letting the spreader fall through the substructure to the ground. Two employees were on the ground below the spreader when it fell. They were looking up to see what needed to be done to allow the spreader to set in place so the pins could be installed. One employee was able to move out of harms way. The other employee was struck by the spreader and knocked into the cellar resulting in a severe injury. A cellar cover was not in place.