Pain going from the back down the leg is a common condition called sciatica. A common cause of sciatica is a herniated disc. The displaced disc will place pressure on the sciatic nerve, which runs down the lumbar spine.
The biggest cause of a herniated disc is bending forwards or to the side and attempting to lift a heavy load. The picture below shows a nerve being pressed on by a prolapsed disc, from the book Sports Injuries and their Preventions and Treatment.
The pain from the sciatica can be felt in the lumbar region, and it will radiate down the leg when pressure is placed on the lower back. You might feel weakness, numbness and pain in the area.
This video shows you how a herniated disc can cause sciatica. If you want to find out more information and treatment check out bulging disc treatment.