Why do these guys ignore me?

I feel that every once in awhile I'll meet a friend's friend and they won't give me the time of day. It actually seems like they're angry towards me; they don't smile when I talk to them/crack a joke, they're not interested in greeting me when they meet up with me and mutual friends, and they'll talk over me when I'm talking to people. It's not a big deal to me because not everyone's going to like you but I just don't understand why they have - what seems like - animosity towards me when I barely know them. Just wondering if there's anyone insightful out there on yahoo answers to explain why they might act like this towards me.