Once you have a fusion of that level and it becomes solid (time to fusion depends upon the method and graft used) you will never have another problem with the fused level. The levels below or above will still have about the same risk or slightly higher risk of herniation as before the surgery. You could go play in the NFL (US) with a one level fusion.
Genetics is the best answer for why this hernation occurred. Trauma and other problems can produce herniations but by far, the most common cause of herniation is genetics.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.