Because shy people don't talk so that awkward silence is more prevalent. Also, shy people may already be nervous so that nervousness spreads.<br />
Not that I'm judging. I'm a shy introvert.
For some people who are more demonstrative or loud, they feel that shy people are just being reserved/quiet because they disapprove of more "wild" or talkative people and think they are being judged for their behavior. I am a talkative, demonstrative, extrovert-seeming introvert so I have experienced this myself.