Autonomic dysreflexia
Scientists think this happens because of afferent nerves.  Afferent nerves are supposed to sense things (like touch) and send messages back to the brain and spinal cord so the brain can decide how to react.  Scientists think that afferent nerves below the spinal cord injury cause blood pressure to get and stay high, by causing the blood vessels to get smaller.  This is part of the fight or flight response.