Spinal stenosis. Spinal stenosis is narrowing of the spinal canal and can occur because of any structure encroaching upon that space. It can be congenital or something that you are born with. It is commonly caused by disc bulging or herniation, facet joint hypertrophy (overgrowth) from arthritis, or ligament buckling. Trauma as you described could lead to a bulging disc or arthritis with time.
Doubtful. It will take many years for osteoarthritis to develop after injuries to cause stenosis. I am assuming that injury as well as stenosis is in low back.