While wearing tight-fitting shoes contributes to bunionette development, doctors no longer believe that shoes actually cause tailor's bunions to form.
The more modern general medical consensus is that bunionette formation has more to do with genetics than footwear (or any other behavioral factor).
Abnormalities of the foot shape or mechanics passed along at birth from parents appear to make some people more likely than others to develop bunionettes. Those with flat feet are also susceptible to bunionette developments.
Outside of genetics, age-related spreading of the toes can also cause or exacerbate bunionettes.