A bunion is a bony bump that forms on the joint at the base of your big toe. It occurs when some of the bones in the front part of your foot move out of place. This causes the tip of your big toe to get pulled toward the smaller toes and forces the joint at the base of your big toe to stick out. There are many theories about how bunions develop, but the exact cause is unknown. Factors likely include: inherited foot type, foot stress or injuries, and deformities present at birth.

Source: Mayo Clinic