Intervertebral disc Disease (IVDD) when the cushioning discs between vertebrae degenerate or slip out (herniate), they can press on the spinal cord, leading to pain or paralysis. Certain breeds, like Dachshunds, are particularly prone to this.
Arthritis age-related wear and tear can lead to the breakdown of cartilage in the spinal joints, causing inflammation and pain, especially in older pets.
Muscle strains from injury or overexertion
Trauma like falls, accidents
Obesity
Further reading: Arthritis in Dogs, Obesity in Cats, Osteoarthritis in Cats

