Not going to turn this into a medical debate about low back pain...but the causes of it include the paragraph below. The most common cause is "idiopathic", ie they don't have a clue why its happening.
"A back spasm can occur after any type of strain or injury to the soft tissues—the muscles, tendons or ligaments—in the spine. Following the general treatment guidelines below and the recommendations from your doctor or physical therapist will go a long way in relieving your pain, and your back muscles should calm down in a week or so.
The muscles can spasm in response to an underlying anatomic problem
If your back spasm does not get better inal problem 1 to 2 weeks, or it comes and goes over time in the same area of your back, you may have an underlying anatomical problem in your spine. Examples of underlying issues that could cause your back to spasm include:
Facet joint osteoarthritis
Degenerative disc disease
Spinal stenosis
Pain from a herniated disc"