Unless you have had a violent accident, your back trouble, whether sudden or chronic, has been developing for a very long time. Muscular tension builds up for a long time before crossing the point of no return and becoming a back spasm. Then, like the straw that broke the camel's back, a small movement is sufficient to trigger a crisis. We return to the quandary of back spasms. Your brain is an organ of conditioning, of learning.
Repetition leads to habit formation and habit formation leads to involuntary habits of tension.
Back pain is a state of nervous tension conditioned through the repetitive strains of life. So the problem is simpler than you might expect. By relaxing those muscles, you can take the pinch off the nerves. If you can't feel how to regulate your muscular tension, you can't feel how to relax your muscular tension. The tension feels like its happening to you, when its really an involuntary habit.