A bunion is a bump of bone around the big toe joint.  There is a common belief that poor fitting shoes cause the problem but the cause is often caused by overpronation.  The overpronation causes the foot to roll-in toward the big toe.  This causes laxity (weakness) in the tendons a and ligaments that keep the toes in line.  Then when you put the foot with weakened tendons and ligaments into a narrow shoe, the increased pressure causes the big toe to bend towards the middle of the foot. 

The bump that you see at the base of the big toes is actually the base of the toe bone sticking out the side.  Over time and without treatment the bump gets bigger and more painful when wearing shoes. 

It is important to diagnosis and correct the biomechanical problem that is causing this problem early, before the deformity occurs.  A custom, prescription orthotic can be constructed to control the pronation of the foot.