I'm stuck in the middle on this one. I agree with Mercedes that it is an insecurity issue. However, I would see it not only as your relationship with the horse, but more so with the horses self experience. If I'm not mistaken, we are not only talking about two mares, we are talking about two fairly young and inexperienced mares, both mares that are low on the totem pole and in a state of social "awkwardness". I have two like that, I'll admit it, a 4 yr old and a 5 yr old, both with a bit of trauma in their past, that have some insecurity issues.
For Jynx, her insecurity is increased because her fight flight ability has been comprimised by being tied up. A horse with any insecurities relies on that mechanism more than anything. When we take it away or limit it, then they are going to be required to be on guard that much stronger. My 4 yr old is like that in her stall with other horses. Out in the pasture, she's bottom on the totem pole but gets along with everyone. Once in her stall, she turns into kung fu woman, making any observers think that she must be one bad lady. Many people immediately assume that since she is in her stall, she thinks that she can be mean to the others and be safe doing it. Looking at my personal horse's weaknesses, I disagree, I think thats putting too much human emotion into it. Looking at the horse, her past, her rank, her experience, I believe that she is explaining herself pretty well. Defensive patterns arise when the horse feels a reason to be defensive. This could be because they are physically limited by boundaries such as a rope or a stall wall, or because there is someone new approaching their herd, be it person or horse.
I do know another horse that shows similar issues towards people that kV is describing. He is not a young mare, but actually an 11 yr old gelding. He lacks social skills so is emotionally comparable to a young horse figuring out where they are supposed to be. He gets increasingly aggressive if his owner has company. I have taken care of him when his owner was away and although his manners were lacking, he was not aggressive. His owner says the same thing kV does, that he isn't bad if there aren't other people around and she is by herself. This horse is very insecure. Even though he has had a lot of undersaddle work, he is pretty physically wrecked from excessive drawrein use and poor training originally, he has minimal turnout, and he is only allowed to make visual contact with other horses, so he acts worse than my young girls by far. When these horses are reliant on whoever is around, they are going to feed on things that you may not even know you are experiencing yourself. We all get a little different when introducing our horses to new people. Your confident horses can shrug it off pretty well, but the insecure horse is like a sponge, not only taking that emotion, but quadrupling it before acting on it. With my friends horse, I found that it wasn't that he was defending her, but rather reacting from her increased energy levels when someone else was in her barn. If I could change her mindset, lower her energy and relax the situation, the horse also relaxed and his defensive patterns slowly became less extreme until they were gone. Keep in mind that these changes that I'm speaking of are so small that some may not even think that they are happening. I speak from experience, I also tend to turn into a ball of nerves when introducing someone new to my horses. My seasoned guys are very capable of blowing me off and taking care of themselves emotionally, but with a sensitive, insecure horse, the tiniest increase in heart rate can be easily blown up by them as there must be something to worry about (again, increased defensive patterns, fight or flight, due to insecurity).
Now, please don't jump down my throat about not knowing the horses or not knowing the people. I fully confess that I myself have contributed to a horse getting overly upset over nothing and I also have some insecure "defensive" girls that have a lot of work to do. So I do agree more with Mercedes on the insecurity aspect. I do not think that they are being overprotective of their people or taking a cheap shot because they can. I believe that it is more defending their space when the space is limited. I also think that they will get better with more experience. The have to be more secure in their skills and stronger in their bodies. Each horse is different, but they all follow the same rule book, sometimes they are just in different chapters.
kV, if I were you, I would do as Spirithorse said towards that behavior. I'm guessing this is when you are bringing people out to see her, which automatically puts her on a pedestal and throws the ball in her court. See if you can take someone out there with you and just ignore her. If she approaches with any type of defensive/aggressive behavior, send her away. Don't accept her into your herd until she is showing acceptable behavior. I hope this makes sense, its somewhat of a mouthful, I apologize.