Disc herniation is often the result of a gradual disc degeneration.
With age there is a progressive disc hydration reduction and so of their elasticity and for this reason they are more exposed to tears or breaking, also as a consequence of light traumas or stress. Moreover, hernias may be caused by weightlifting when loading the back instead of legs, while they are less frequently caused by traumas, i.e. a fall.
It is clear as some factors may induce disc herniation more than others.