I would like to preface this article by stating that this is merely information, not the cure to your pain. The reasons for lower back pain are numerous so please do not fall into "WebMD-itis," the illness of diagnosing yourself with a fatal illness after reading the internet. I am trying to create self-awareness, not hypochondriacs.
"Everything happens for a reason."
Sometimes this sounds like just a cliché. Sometimes you can logically see this is true. For back pain, there is definitely a reason for it. One does not magically get a "bad back." There is a reason for it. What's the reason? Well… it depends! There are so many things that could go wrong to cause lower back pain. To be clear, when I am referring to the low back, I am speaking of the lumbar spine and the (tail bone) sacrum. The lumbar spine's job is to primarily stabilize the surrounding areas for the torso, allowing the hip and thoracic spine (upper back) to move. When complications arise, the hip and (upper back) thoracic spine lose the ability to travel through the range of motion your joints are capable of getting to.
One of the more frequent causes of lower back pain for runners is continuous running without a strength training program that optimizes structural balance. Structural balance is important to reduce the probability of repetitive injury. When an activity calls for performing the same muscle action repeatedly, muscles may stay in a shortened or lengthened state. For example, if while running, you constantly shorten your hip flexors (the front of your hips), they could stay shortened during everyday life. Tight hips can overarch your back, putting more stress on your lower back. Although this is a normal pattern for runners, there are just as many runners with back pain stemming from other causes. A competent strength coach/trainer should determine your weaknesses and provide a sufficient strength training program highlighting general strength and structural balance.