Why do smart people get so quiet around less intelligent/ experienced people?

I don't find that. In fact the most intelligent people know how to carry on a conversation, most importantly they know when it's time to listen quietly without interrupting. 

I think that would depend on the person themselves rather than that tendency being something that correlates to a person's intelligence. Though I don't know why it would be for some smart people. I'm not smart, but there are cases where I know more about a certain thing or am better skilled than another person, and I love explaining and introducing it to them if they're willing to listen. Though I tend to go on tangents, and sometimes I have to be interrupted for them to tell me that they have no idea what I'm talking about. Maybe that's why? Though that sounds a bit arrogant to do--Refuse to talk to some people as much as you would with perceived equals because you think that you're so smart that they can't understand. If someone tells me that I'm going to fast or talking about something that they don't know much about, then you just have to explain more and go slower. It's a nice feeling to me to see people interested in new topics or even just getting some information on something that they normally wouldn't study. I myself like to often pick up books on obscure, often dull things that I wasn't interested in before to immerse myself in new information.

Depends on where I am and who is part of he conversation.  If asked a direct question, I will respond.  Otherwise, unless someone is about to drive off a cliff, I seldom say anything irrefutable.

I tutored often when I was younger.  It is essential to meet the student (whether of math or life) where he is and show him how to move forward---it is not my place to force him into a specific way of thinking nor to denigrate his experiences.