How does flat feet cause lower back pain?

Via the legs: The lower extremitie acts as one big unit. The foot being flat changes the way the shin bone sits on the ankle thus altering the angle of the thigh bone the hip then swivels and rotates and throws off the hip/sacrum interface. Once this occurs the lumbar spine is thrown off and the low back muscles spam causing pain. All this is done to keep our eyes level in order to maintain depth perception.

Poor mechanics: Not everyone with flat feet will get low back pain. The feet, knees, hips, and low back are all connected like a chain. If you injure or change the mechanics of any part in the chain it can hurt the other parts. Some people with flat feet turn their ankles in which can result in poor alignment through the knees and hips ultimately resulting in low back pain.

The flatfeet: Throws off the biomechanics of the structural skeleton putting increased strain on the lower back.