I ruptured a disc just by sneezing. There was no warning at all. I never had a day's worth of back pain until this happened. How do you explain this?
It's not unheard of that a simple act such as sneezing or coughing can result in a disc herniation. Most likely the disc had been weakened to such a point that physical disruption occurred after what seemed like a minor incident.

But the truth is that discs age and degenerate right along with other body parts. Years of twisting, bending, and compression slowly cause structural changes in the disc that can eventually lead to a break down.

Studies show that a combination of factors can keep a disc from healing. Such things as age, genetics, poor nutrition, and decreased blood supply to the disc may impair the healing process.

Your sneeze was like the proverbial "straw that broke the camel's back." It was just the final stress on an already weakened disc that led to enough damage to cause disruption and pain.