We don't have any public transportation down here, but I do know this is a guy's passive-aggressive way of saying, "I need my space." They hope that by expanding out their elbows and knees, they have effectively claimed control of that area and now, if they do happen to sit more relaxed and less expanded, there's this bubble that they have created.

It's annoying I know, but nothing tries to put more space between other human beings and himself than the American guy.