Personally it was the stutter. I vividly remember not stuttering, up to age 5. I was a bubbly kid by all accounts, I loved talking and did it a lot. I also remember being mercilessly bullied when I developed it, especially by my family. As a preteen and teen I still had a measure of extraversion left and tried to socialise but by the time I was 12 it was so bad I was practically mute. Up till now I feel a deep longing to mix with groups of people but it's always overpowered by the deep-seated fear of embarrassing myself.