This is very common. Most of my patients who come to see me for any sort of back or neck pain tell me it started suddenly, and they do not recall injury, falls, accidents, etc. What happens is that certain areas of the body are under stress for many years till they exhibit symptoms or pain. So to answer your question, there has been stress on the joints and nerves of your lower back for many years, and it is only now starting to create problems for you. Some causes are years of poor posture, especially with desk jobs, physical labor from work, poor lifting/bending techniques, and emotional stress. I hope this helps! Find a chiropractor to get evaluated to see if an adjustment can help.