The lower part of your back is meant to help support your upper body stay upright. Every time you bend at the waist, the lower back takes on the weight of he upper body and has to hold it against gravity – this means more force on the spine. Even if you are just bending to pick up a pencil, there is an unnatural force working on your back. Then, if you bend over and lift something, the lower back has your body weight plus the weight of the object you are holding.
If you want to lift correctly, reducing the risk of injuring your back, the proper way to lower yourself is by using your legs. Your thigh and leg muscles are much stronger than your back and are meant for bearing weight. By keeping your back upright, bending your knees, and then lifting, the bulk of the weight is on the legs.