The human body is an intricate machine and everything is connected so when something goes wrong in one area, it can cause problems in other areas. The back carries a lot of the stress in the body so when there is a problem with the hips, knees, or feet, the back can bear at least some of the brunt of the pain and other effects.
This is particularly true with foot problems. When the mechanics of the foot are off it can throw off the alignment of the entire body. Overpronation and oversupination in the foot can lead to serious and chronic back pain.