fused vertebrae

My 75yr-old father-in-law has recently been told by his GP that he has some vertebrae fused together in his lower spine. He has not had any injury or falls that we know of. I know some people are born with this condition, but this is not the case here. The diagnosis was made from an X-ray, and followed several weeks of back pain and limited movement. I am wondering, how can vertebrae just fuse together like this?