Bunions are caused by uneven pressure on tendons and joints in the foot, according to the Mayo Clinic. This pressure causes the joint of the big toe to become unstable and form a knob that sticks out from the side of the foot.
There are varying things that can cause this uneven pressure, as explained by the Mayo Clinic. Wearing shoes that do not fit or shoes that manipulate the feet, such as high heels, may cause bunions. Bunions might also be genetic and inherited. They may also form following a foot injury or be present at the time of birth, and they are associated with other conditions such as arthritis.