There are many reasons for a horse to pull back while tied. Most likely something spooked the horse and her natural flight instinct kicked in to flee, but she hit the pressure from the halter and lead rope and pulled back in an attempt to escape the pressure. We often wonder why a horse pulls back; the horse is simply pushing back on the pressure of the halter and lead rope in an attempt to flee. Horses are prey animals and prey animals instinctively push into pressure to escape. For example, when a lion attacks a zebra, the zebra will push back into the lion in an attempt to break the lion's hold/grip so the zebra can escape and survive. This is why it is paramount to teach our horses to give to pressure. To survive in the human world a horse must first and foremost understand to give to pressure.
Since a horse's natural instinct is to push against pressure it can be challenging to teach a horse to go against their nature and give to the pressure. However, with the proper tools you can teach any horse to give to pressure. The tools needed are a halter and lead rope. Keys for being successful in teaching your horse to give to pressure are timing, release, and reward.
Remember to also desensitize your horse to as much stimuli as possible. Horses spook at rapid movements and different kinds of sounds. The more the horse is exposed to, the more comfortable she will be when the unexpected happens and the sooner she will relax and reduce the risk of pulling back.