What causes severe buttock pain down leg?

Nerve irritation. Buttock pain that radiates down a leg most likely is the result of a nerve irritation. It may be secondary to a herniated disc, foraminal stenosis or muscular compression as in the piriformis syndrome. Would recommend that you see your pcp for an evaluation to determine your exact diagnosis.
Pyriformis v disc. This sounds like sciatica which can occur due to a disc herniation in the lower back and not cause any back pain or be due to a hip muscle known aa the pyriformis irritating the sciatic nerve which it straddles in the buttock region mimicking a disc herniation pattern.