I think some people are afraid to make their own choices, "decide" where to sit, prefer to sit where someone else is sitting so no one will question their choice, get angry at them and, they have an excuse, "well, she did it first!" I feel a little sorry for them, they're more anxious than I am and, like you say, try to make conversation, I think so they don't feel as "alone" and can identify and connect with their allies, there's more safety in numbers you know :-)

I think it can be kind of creepy though too, and annoying (and, for me, embarrassing because I try to snub them and feel badly for it or at least do minimal talking and then resent it/them). I hate having to sit next to someone else, always choose the empty seats, as I'm afraid they'll reject me, get up and move or be rude, etc.