There are many causes of a herniated disc, but it all comes down to this: your intervertebral disc (the cushion in between your vertebrae in your spine) pushes out or bulges or even ruptures. This very contained disc (we have an article to help you learn more about intervertebral discs, should you want to) starts to take up more room than it should, and it can, as you're probably well-aware, cause you a lot of pain.
Pain from a herniated disc is often the result of daily wear and tear on the spine. This is also called degeneration.
Our backs carry and help distribute our weight, and those intervertebral discs are made to absorb shock from movement (such as walking, twisting, and bending). Because our discs work so hard to help us move so well, they can become worn out over the course of time.
The annulus fibrous (the tough outer layer of the disc) can start to weaken, allowing the nucleus puplosus (the jelly-like inner layer) to push through, creating a bulging or herniated disc.
A herniated disc can also be caused by an injury. You can herniate a disc in a car accident, for example: the sudden, jerking movement can put too much pressure on the disc, causing it to herniate.
Or you can herniate a disc by lifting a heavy object incorrectly, or by twisting extremely.
It may be that an intervertebral disc has been weakened by wear and tear (degeneration), making it more prone to herniation, should you experience a traumatic event.
Or it could be that your disc has become so weakened that something that doesn't seem like a very traumatic event can cause a herniated disc. This is the case when people herniate a disc sneezing (it does happen!). A sneeze doesn't seem like a traumatic event that could lead to injury, but if you have an already-weakened disc, then the sudden force of a sneeze can herniate a disc.
There are 4 stages to the formation of a herniated disc, as shown here:
Progression of Herniated Disc