In truth, a bunion is formed due to an instability in the feet and mechanics that causes the first metatarsal bone, the bone before the big toe, to shift over. As it is shifting, tight shoes could make it happen more rapidly, however it would continue even in well-fitting shoes.
If you notice the start of a bump on the side of your foot, don't wait to get it checked. When a bunion is noticed and treated in the early stages, the faulty mechanics can be addressed with a custom orthotic to restore stability to the foot. This can prevent a bunion from forming or stop it from progressing.