To answer the question, we have to differenciate between 15% with specific backpain (caused by a clear, identifiable structural cause) and 85% with unspecific backpain. In the huge unspecific group the best specialist is not able, to find one clear cause. And even, if we believe that we found the cause e.g. in the MRI, in most cases at has nothing to do with the real cause. Just to demonstrate, what I mean: Already in the late 80-ies studies found out surprisingly that in more than 60% of all totally painfree, healthy subjects the author could identify at least one unless more discprotrusions or even discprolapses!
In the specific causes the most common one is the disc-disease. In 100% of acute disc prolapses the main cause is genetics, lifting something too heave or anything else can elicit the process, but never cause it!
Most of the other specific causes like tumor, inflammation, severe neurological deficite, fracture,....rarely happen.
The most painreceptors are in the ligaments, tendons, capsules, muscles, fasciae - no diagnostic procedure is able to show us a picture of activated painreceptors or irritated soft tissue. Normally we never load these structures directly, because our bodyown musclecorset protects them. But if we for what reason ever develope some ache or pain in the locomotor apparatus, we automatically develop relieving postures and -movement as well as a pain-avoidance-behaviour. For example, if you have some uncomfortness or ache in your lower spine, you will automatically begin to bend back and sidewards less often and far, because in these positions you would compress the spinal joints and make the nerveholes narrower. By not using these muscles, they from the first minute on begin to get weaker and less skilled. But they are your only protection against hanging in your ligaments and capsules, what definitely is painful. The further increase of the pain will then intensify your relieving postures etc.
It sounds rather unspectacular, isn't it?
But the intensity of the pain has rarely something to do with the severity of pain!