Typically this is normally a reaction to you accidentally hitting the horse in the teeth with the bit when unbridling. If you are doing this, then you could practice proper unbridling with another horse and then re-teach your horse not to fear unbridling. In this case, your horse probably fears the pain of unbridling and throws up her head to escape it. This is the normal cause, but there could be other causes as well. Do you know when her teeth were last done? There may be a problem with her teeth that makes unbridling uncomfortable. Other than those problems I can't think of another reason right now... I would suggest maybe getting a vet out to check her teeth, as well as seeing if you are accidentally hitting her teeth with the bit when unbridling. Please be sure to solve this situation fast though, if a horse gets used to doing this it can take a long time to re-train them. I hope this helps!
Once I ruled out any pain issues I would try this. Slowly remove the bridle one baby step at a time. Such as one ear, then the other, then slowly start sliding the bridle down the nose. What I am looking for is the point that the horse starts to react and throw her head up. The minute she starts to react, put the bridle back on and take a step back. She knows that you are the one that takes it off and she needs to learn that if she does not stand still, it does not come off. Keep doing it until she lets you take off the bridle without a fight. If this is just a bad habit or a reaction to something that happened in the past, it may take a while to correct, but have patience and don't get frustrated.