See below. That there is something, bone, disc material, tumor, something reducing the diameter on the spinal canal and causing pressure/compression of the spinal cord and that the compression is acute or rapidly progressing in nature.
Sudden pressure on. That there was a sudden pressure placed on the spinal cord which depending on the degree of the pressure and its cause can affect the function of the spinal cord. Any deformity or spinal instability can also play a role as well as the age of the patient and their underlying health. The spinal cord in general does not tolerate sudden changes well.