I think you'd have to differentiate shyness from social anxiety. I wasn't exposed to people early on. I was protected from everything. Everyone in my family was loud and spoke for me, so I didn't have to. So I was reserved around new people. I was very shy.
It became unhealthy, I think, when it started making me feel lonely, and I was unable to fix it. I never learned to develop social skills in a natural way. Wondering why I was lonely, could only assume there was something wrong with me. That insecurity is the cause of social phobia, IMO.
As I see it, natural introversion, and shyness, can cause a predisposition to a social anxiety disorder, and complicate it as well. But at the same time, some people with social anxiety would never be seen as externally shy.
Just my thoughts.