I used to be loud as a kid but that all changed when I started secondary school..Over the years I just started getting more and more socially anxious and started avoiding social interactions as much as I possibly could, Even ended up having no friends and not going out because of it..
14-21 years old I was depressed all the time because of SA, Now I've accepted the fact that i might not be able to interact with people without feeling anxious, Have friends or be able to have a girlfriend etc.. I honestly don't care anymore I'm just going to focus on things that I like doing / makes me feel happy.
To answer your question, The reason how it started is because of school, School is filled with bullies.