What is cervical myelopathy due to?

Several things: One of the most common is spondylosis: the natural degenerative (aging) of the neck, which narrows the space for the spinal cord. A large disk herniation (rupture) can also do this. Sometime ligaments within the channel containing the cord can thicken, occasionally becoming bone, and compress the cord. Pus from infection and tumor can do this. Radiation can cause myelopathy as can some infections.

Multiple reasons: A large disc herniation, narrowing of spinal canal due to degenerative changes, calcification of spine ligaments with enlargement, spinal trauma, instabilities secondary to prior trauma or degeneration, cysts, tumors, epidural abscesses, syringomyelia, post radiation, to name several reasons. Most times the spinal canal is too narrow for the spinal cord.