Maybe you exude an air of niceness that makes these people want to talk to you. If it really annoys you that they keep talking, maybe just be a bit more direct in trying to say you have to go about your business. I know that's pretty hard with some people, but you have to think of yourself as well and set some boundries.
I am personally quite amazed by people who "can't stop talking" as I can't really do that myself. Some people wear their thoughts on their sleeves. It may be a thing for older people more then younger people though. Older people are often not so uptight as young people, who still have to 'make it' in the world, and are very concerned about how they come across to others. Anyway, I do feel comfortable around these people for some reason. I guess I am a good listener and like other people taking the incentive.