One of the biggest underlying causes of social anxiety can be the belief or feeling that a certain outcome will mean that "I" will die. Not physically, but psychologically. 

If the whole room is judging you in a certain way, or laughing at you for something, it is easy to feel as if you are shrinking down to nothing without your old identity to rely on anymore. We feel as if something like that will mean we will lose who we are, as if we are dying in some way. And that can be scary...