Bunions are caused by a biomechanical abnormality, where certain tendons, ligaments, and supportive structures of the first metatarsal are no longer functioning correctly. This biomechanical abnormality may be caused by a variety of conditions intrinsic to the structure of the foot–such as flat feet, excessive ligamentous flexibility, abnormal bone structure, and certain neurological conditions. These factors are often considered genetic.

Bunions are commonly associated with a deviated position of the big toe toward the second toe; and the deviation in the angle between the first and second metatarsal bones of the foot. The small sesamoid bones found beneath the first metatarsal (which help the flexor tendon bend the big toe downwards) may also become deviated over time as the first metatarsal bone drifts away from its normal position.

Arthritis of the great toe joint, diminished and/or altered range of motion, and discomfort with pressure applied to the bump or with motion of the joint, may all accompany bunion development.

Although wearing shoes that crowd the toes does not cause bunions to form, it sometimes makes the existing deformity progressively worsen and symptoms may arise sooner.