What causes pain in the lower back and goes down the leg?

Siatica. Piched nerve in low back at the bottom two levels disc herniation or bone spur.
A pinched nerve. The most common reason for back pain that shoots down the leg is a pinched nerve which may be due to: 1. A herniated disc 2. Spinal stenosis 3. A slipped vertebra (spondylolisthesis) 4. Other reasons (tumor, infection, etc.) seek medical attention if there is progressive worsening of pain, numbness, weakness, tingling, or bowel/bladder issues.