I hate to say this but age is the #1 factor, most common in people over the age of 50. Age is a a major factor because as we get older the disc height diminishes. Our disc is filled with mostly water but with age the water content decreases.
In addition to the disc degeneration with age, we also start to develop osteoarthritis (OA for short.) OA occurs when the cushioned ends of bones,(cartilage) wear down increasing friction between the bone ends. That friction over time causes inflammation, pain and stiffness. With significant stiffness the friction can lead to formation of bone growth known as osteophytes which occupies the space where the nerve root is, hence the unwanted symptoms.
Herniated disc. I have a entire page written about this so check it out!
There are some patients that I have met that has been provoked by trauma but in that case there were some degree aging process involved but exacerbated by abnormal physical stress. When I say physical stress it can be fractures, disc herniation, etc.
Tumors. Again growth of something that should not be there, takes up room where the nerve roots lay. But that is all I will say about that. Please do not jump to the conclusion that you are experiencing this secondary to a tumor! You are more at a risk to experience the symptoms due to the causes mentioned above. This is not my area of expertise!