The deep spinal muscles are individually quite small, so are particularly vulnerable to sudden overload or repetitive motion. The only time they are relaxed are when you are lying down or standing perfectly upright with your weight evenly distributed on all sides of your spinal column.
So if you start to slouch, shoulders hunched, head hanging forward, these small muscles must work continuously. This in turn creates trigger points within the muscles, which weakens them and prevents them from working properly. They can, in extreme cases, pull the vertebrae out of place and damage the disks, causing numbness in parts of the body served by the nerves. They can be the cause of scoliosis, where the spine visibly bends to one side. They send pain forward into the abdomen, or downwards into the buttocks.
Back pain can, however, be banished by trigger point therapy in a high percentage of the time.
There is also a number of self help things you can do, such as strengthen your abdominal muscles so your back muscles do not have to work so hard when you bend and twist and move about. When reading, prop your book up on some kind of stand so that your neck and back are straight. Make sure your computer screen is not too low. You should be able to keep your head level when looking a the screen and sit straight on to the screen. Avoid heavy lifting unless you are able to keep your back straight and square on to the object and take the weight in your legs.