Horses kick...it is a part of life. Even a passive horse who is NOT known for kicking can kick out and injure another horse or person. They are animals and have minds of their own.
I do not believe you are liable or responsible for any of this. Horses get tied up ALL the time for tacking and ANY horse could have kicked out. You gave fair warning (providing she heard you). If she wanted to get by all she had to do was ask you to move your horse. From the sounds of it there were plenty of other ways she could have gone that didn't include going behind your horse.
At a lesson barn I used to ride and work at we had a known kicker. He was a lesson horse and a great one under saddle. It was understood by everyone that he was a kicker and nobody was to walk directly behind him. There were young children at this barn and even they undertsood that they could not go behind this horse...their parents also made sure of it. If someone is stupid enough to walk behind a horse they don't know, or that they know to be a known kicker, and gets injured, that is entirely on them.