Dog will not let me walk behind her when I ask her to stay

Hi guys.
I have a jackrussell mix that I have had for 3 months. I have trained her to do the four basic commands: sit, stay, come, and lay down. We have been working on stay in different circumstances such as not being at home or while someone is squeaking the squeaky toy, or me walking into another room she does all of these well unless I walk behind her. None of my other dogs have ever had this problem. My question is why would me walking behind her cause her to get up or turn around?

I would say that you are rushing it - why should she not want to be able to see what you are doing?. Even competition dogs are allowed to face in the direction that their owners are moving to. If you specifically need her to only face forwards while you go behind her, I would break the exercise down so you are only moving a single pace back at first - then two etc. etc. Do remember to reward or praise for getting it right at every stage, and treat any failures lightly in case it becomes a source of anxiety for her.

I will try to explain how to get your dog to sit with her back to you.
Sit on a chair or settee with her facing you. Then with some treats in your hand get her to follow the treat with your arm outstretched going in a semi circle so that she follows the treat and turns round so that she is facing away from you. You are still sitting on the chair/settee. Ask her to sit and give the treat. She should be sitting with her back to you and eating the treat in the hand that is in front of her nose. You can then slowly stand up with her still sitting with her back to you. Keep telling her she is good for sitting. In her past life she may have been kicked or been afraid of dogs coming up behind her to do something that she didn't like. Or even patted on her behind to rigorously. This will take time so don't rush onto the next stage of tryng to walk round her until she is comfortable with you doing this first bit. Eventually you will be able to stand up with her still sitting and then you can move to either side of her and then in front of her and then all the way round her. If she starts to get up go back to stage one. Hope that all makes sense.

As to why she does it? She probably feels more confident if she can see the direction you are going in. On the other hand, it could be for any number of reasons

Yes. I agree with reasons stated above. I think you are rushing her. Give her some time. If it continuously is a problem then I would try training her to stay there. I believe she is afraid still. If it doesn't work contact me.