<p>Maybe it has something to do with what’s available to concentrate on. People can concentrate on a discussion, an object, another person or group of people (a one-sided observation/judgment, as opposed to discussing a person’s history or interests, for example), or their own inner thoughts. If you’re in a group of people that doesn’t consist of a study group or anything like that (so we assume that concentrating only on your own inner thoughts would be considered socially strange), and the topic of discussion ends and there’s no immediate object or outside person/group interesting enough to concentrate on, that leaves only being able to concentrate on other people in the group. Which is usually awkward for people not dating or on that best friend status.</p>
<p>Also, people are always critiquing themselves, and sociability is considered a skill of sorts by most people, so they might see it as a temporary failure on their part, which they become embarrassed about.</p>
<p>Also, standing around doing nothing is weird.</p>