I began self injuring as a way to protect my family at age 3. Sounds strange, but I'd hurt myself to get my dad's attention off which ever family member he was going off on. Once his attention was on me then he'd be angry with me and he'd beat me instead of my mom or siblings.
As I grew up, the SI became a coping skill. I have done many forms of SI. Cutting started when I was 13, and eventually it took over and became my main way to SI.
I have been diagnosed with attachment/abandonment disorder, dissociative disorder not otherwise specified, depression, anxiety, PTSD, eating disorder, and a few others are being questioned.

As someone else has already explained, anybody can SI, you don't need to have another disorder to SI. I have a friend who SI's more frequently than I and she has no mental health disorder, it is just how she learned to cope and now it has turned into an addiction. SI is actually considered an addiction.
This was a good question. Thank you for asking it.

Be safe,

Celtic