There are two broad types of back injury: sudden and cumulative. Even if the onset of pain is sudden, the injury may have been developing cumulatively, beneath our awareness, for some time.
Pain is a bodily system like the respiratory system or the cardiovascular system. Its function is to warn of us harm or potential harm. Like the other systems, the pain system can develop dysfunction.
Pain and tissue damage are not the same. Many factors other than tissue damage affect our experience of pain, especially if we've been in pain a long time.
Prolonged exposure to pain decreases our pain threshold.