My Percheron did the same thing initially. I don't know if it's a draft horse thing or not, but have heard similar experiences. Once I went to free longing, got all the respect I wanted. He was just protesting being on the line. I don't know if it hurt him, was uncomfortable or he just plain didn't like it, but I didn't want it to be a continuous fight.
So one day I just took him off the line and was going to just chase him around the arena, he ended up doing perfect circles around me in both directions. Stopped when I asked, ears forward, no butt turns no nothing.
As you know, draft can very 'vocal' with their snorting, growling, ear pinning, tail swishing! I think 'something' could be going on, especially since she is elevating her behavior. It may be that she just doesn't want to do it. But I also know most of the draft riders I've talked with have the same issues - the horses don't do that great on a line. And the line may very well remind her of the accident.
Try free longing, see what happens.
Unless it weighs a ton... it's just a horse. Draft horse motto.