I think your born with pre disposition to develop social anxiety. Just like a lot of people with schizophrenia have it in dormant state and it never really activates. Once you have the predisposition then life experiences, your looks and social status shape weather you develop SA or not.
We know It can be developed from how you were raised. How much nature plays a role is a bigger mystery. I've read many articles on this subject and concluded that both do their part in screwing you up.