Studies suggest it may be a little bit of both. Heavy lifting is a predictor of future episodes of back pain. This could be the case of mechanical overload: lifting more than the spine is designed to handlt. Back pain is more likely to go away and stay away when there's a decrease in the workload.
There may be a fear factor here in that a person's back starts to hurt when the person avoids certain movements in order to prevent injury. Studies show greater psychologic distress in patients doing heavy work compared to part-time workers or workers on light duty.