Explain to me why Paget disease could cause abnormal pressure on a spinal nerve?

Thickened bone. In paget's disease, the bones break down, then try to rebuild themselves. The problem is, the new bone formation is abnormal - the bone is laid down haphazardly and is weaker and thicker than normal bone. If this happens in your vertebrae, the thickened bone can put pressure on the spinal nerves.
Bones I say. Pagets disease is a disorder which involves abnormal bone metabolism, affecting both new bone formation & acceleration of bone loss. This leads to malformed skeletal structures. If one of those structures is the spinal canal nerve root and / or spinal cord compromise can occur. So yes, paget's can & does cause nerve problems.