I can't say for sure, but I'm fairly certain mine started due to my home life as a kid. There was so much arguing and violence in our house that I remember thinking I was watching Tom and Jerry in real life. I spent at least half my childhood hiding from my parents. I wasn't ever bullied though - I just avoided people habitually once I started going to school and it spiraled downwards from there.
It seems for the majority of us that some kind of life trauma was the trigger. I don't think I've ever heard anyone with SA say they had a great childhood or adult life before they developed SA. For me it was my mum suffering with post natal depression and taking it out on me as a baby and then sexual abuse by a relative at the age of 6. I was never bullied though.