Muzzle punching?

[SusanJ]

Hi, my name is Susan. I adopted a Basenji about 2 years ago. She is a very sweet, affectionate dog, but after about 6 months with us, she started "Punching" people in the nose with her nose. Hard. It hurts. She only does it once, to each person she meets. She did it with her teeth showing to the last guy, and cut his nose pretty badly. That completely took me by surprise, because I have never seen her take to a stranger the way she took to him. He sat down and started petting her, she was relaxed, "smiling", clearly enjoying his attention and even giving him that "I love you" look, then he turned his face down to look at her and BAM! She just nailed him. It was as if she was just lying in wait for the opportunity. After that, he didn't want anything to do with her, but she went about her business like nothing even happened, and didn't bother him again. Can anyone help me understand why she does this? Is it a game? Or some sort of test? I should also mention, that she only does it around me. If she is out with my sister, she doesn't do it. I am an assertive person, but my sister is an extremely dominate sort of person. Am I just not a strong enough leader for her? Maybe she thinks she needs to let people know she will protect me? Is there anything I can do about it? I don't want to have to keep her away from people, I have always raised well socialized pets, and I need to understand how to keep her well socialized.