Telling someone bye can have two different stories. They might be saying bye because they or the person they are talking to might have to go. Or they might be saying bye because they are not okay/upset/mad, I know whenever I'm not okay what I say is "well bye I guess" or "well bye" or "bye I guess" so whenever I say that I expect the person to ask what's wrong or something else. I don't know why but I just do that.