I can think of lots of situations where people self-segregate along economic or cultural lines, without much in the way of external pressure to do so. I'd imagine it has a lot to do with viewing oneself as part of a group, and wanting to spend time with people whose interests and background are similar, have similar values, etc.

I'd think this is something that academics have studied already; it shouldn't come as a great surprise that when people are released into some new environment they gravitate towards others similar to themselves.