An employee and co-driver prepared to pull a trailer to a dock door. Before pulling this trailer, the employee went to make sure that the trailer's door was closed. The employee ducked under to check and leaned into the adjacent bay, which was empty at the time. Then, another driver backed a 53-foot long trailer into the previously empty adjacent bay and did not see the employee. The employee was struck by the trailer, resulting in two broken clavicles, which required hospitalization.