Low back pain is the most common complaint among amateur and professional golfers. The modern swing puts a lot of torque on the leading hip. If you don't have enough rotation in that hip the force of the motion gets translated through the hip to the sacroiliac joint or the low back.
Several studies show golfers with more outward hip rotation than inward rotation are more likely to have back pain. Experts in preventing and managing golf injuries advise stretching the hip and lumbar spine. There are no studies to prove this works yet, but until proven otherwise, it's considered good advice.