There is a difference between being introverted, shy and socially anxious. Not all introverts are shy and not all socially anxious people are introverts believe it or not. I think people can sense the discomfort of others. If someone is socially anxious around someone they just met, the non anxious person senses their discomfort and can read it as rejection or just feel uncomfortable themselves. It's nature to feel less favorable towards someone who makes you uncomfortable. You also seem to have your own preconceptions about extroverts which perhaps they pick up on.