The human body was not designed to sit....well at least not for 60+ hours a week at work and then on a soft lazy boy at home.  The spine works best with a combination of sitting, standing, squatting, walking, and lying down.  Doing too much of any of the following can cause considerable distress to your lower back.

1. Decrease in Flexibility - Sitting for long periods of time makes the back, joints, muscles, and connective tissue very tight.   This can lead to back pain as a result of rigidity.  Think of being on airplane for 4 hours and then standing up to get your bag from the overhead compartment....essentially you are doing that every day to your body.  

2. Decrease in Blood Flow - If you have been having back troubles sitting won't help.  The spinal discs heal through movement, a result of transferring nutrients through the compression and decompression of your spine as you move.  Think of this as a sponge, if you are squishing it all day no water will ever find its way in.

3. Bad Posture - Mom talked about this one forever...turned out she was right!  Staying in a fixed position can change the shape of the spine, sometimes progressing to a stay of permanent alteration as the bones mold to your chair.  Good while in the chair...bad everywhere else 

4. Decrease in Core Stability - If your lower back is not stable, you will suffer the consequences.  Poor motor control and stability from the abdominal and lower back muscles can result in chronic back pain, herniated discs, muscle spasms, or sciatica.  One of the best ways to address this is continuous walking.  Spending all day in a chair can make simple tasks such as bending over to pick up your shoes or reaching into a cabinet very difficult.  

This article by Julie Deardorff, Sitting Too Long Puts Stress on the Spine, gives some pretty good advice.  Take a look.