An employee was standing on an inclined mobile ladder using a pipe wrench to remove a shaft from an auger on a feed haul trailer. The wrench slipped off the shaft, causing him to lose balance and stumble backward down the ladder. He was unable to grab the handrails on the ladder to stop his fall, and he landed on his tailbone on the floor. His head then fell back and hit the floor. He suffered a lower back contusion and a head laceration.