It appears that something happened to your mare to cause this. If it is not some physical problem, then it may be a stable problem. My mare went through a situation somewhat similar. She was never a problem to lead in and remove her halter, and touch her head until one day. I went to put my hand on her head and she flew back. This was a barn management problem. From what I figured out, someone had flicked or threw the lead at her head.
I recently purchased a “dually halter” from Monty Roberts. You can contact him on line. He is located in California and is noted for his humane training methods. His halter is safe and amazing. I use it on both of my Arabian horses. I do not want to have my animals hurt and this halter will release. The principal is simple and the horses respond very well.
I would not leave the horse alone to figure things out. This is where injuries occur, especially when you know there is a problem.