I have sudden onset low back pain with radiation of pain down the leg, no injury. what's causing this?

Disk perhaps: A common cause would be a disk herniation (rupture, slip) in the low back. It can also be caused by nerve irritation by bone spurs in the low back. Less commonly, disk injury without herniation can also cause this type of symptoms. Often such problems will improve with time. Seek help if you have problems emptying your bladder or you have weakness in the leg muscles.

Pinched nerve: People often develop a pinched nerve in their back or neck for no reason. They often wake up with the symptoms and this is usually due to a bulging or herniated disc and is more common in younger folks. See a doctor if the pain is severe or you have weakness/numbness or bowel/bladder problems. You may need MRI if not better soon.