The two most common reasons for a disc to protrude or herniate are trauma and muscle imbalances. Muscle imbalances lead to what are called "postural dysfunctions." This images show an example of such a dysfunction: the left person has a normal posture, while the person on the right has an imbalanced one:

If it is not caused by trauma, a lumbar disc protrusion does not happen overnight. You may be come symptomatic very quickly, but it takes a long time for conditions to be right for the nerve to be put under enough pressure to cause pain.
Go to FixMyDiscDisease.info to read more about muscle imbalances and their causes.