Many possible causes: tight footwear, genetics, mechanical i.e. flat footed certain illnesses - arthritis
Treatment options
Footwear - need a wide, deep and rounded toe box. With straight edge along big toe border
Padding to protect skin also use a moisturising cream daily if you have hard skin
Painkillers if joints, arthritis or painful
Simple arch supports although there is little evidence this will prevent deformity worsening
Digital sleeves, spaces or toe props can help cushion and improve alignment. The Internet is a good source for products.
Strapping on YouTube the ballet blog with Lisa Howell shows a good bunion strapping technique
Foot straightening stretching exercises aim to strengthen small muscles in around toes and arch to help maintain flexibility and mobility has good selection of self-help videos
Pain relief - steroid injection acupuncture therapies can help
Surgery: assessment is concerned with level of pain and disability requires orthopaedic surgical referral from your GP or HCP