Employee was operating Cat 990 Loader to clean around a railbed. As he was backing up and looking backward, he ran over uneven ground which jostled him. He claims to have heard a pop in his lower back and that pain followed. NOTE: Incident was first reported as a first-aid-only case on 10/24/13. Management was informed of a 12/18/13 visit to a physical therapist, on 12/19/2013.