The underlying cause is a deformity of the joint at the base of the big toe. The deformity is called hallux valgus. In this deformity, the joint develops a prominent sideways angle. Due to this deformity, the bones of the big toe are pushed towards the smaller toes.
The skin over the angled joint then tends to rub on the inside of shoes. This may cause thickening and inflammation of the overlying skin and tissues next to the affected joint.
In most cases, it is not clear why a hallux valgus deformity develops. There may be some hereditary (genetic) tendency to have a weakness of this joint. In some cases, it is associated with a joint problem such as osteoarthritis or rheumatoid arthritis.