The best way to prevent your horse from pulling back while tied is to teach her to give to pressure. There are many exercises out there to teach a horse to give to pressure. Here are a couple of examples.
Stand on one side of your horse between her head and shoulders. Place one hand over the halter at the poll, and your other hand over the halter at the nose band. Apply steady downward pressure until your horse lowers her head, even if it's only half an inch. Your horse will learn the pressure goes away when she gives into the pressure by lowering her head. If she pushes back into your hands, continue to apply steady downward pressure until she relaxes the slightest bit. If your horse is really tall, try to keep your hands on your horse for as long as possible or stay with your hands raised towards her head until she relaxes and lowers her head the slightest bit. Repeat a couple of times, then do the same exercise on the other side of your horse.
Stand with your back to a fence line facing your horse who is about two to three feet away from you. Signal your horse to move to your left or right and ask her to parallel her body up to the fence. The goal of this exercise is for your horse to move from side to side on a half circle (as if she were tied to you, the fence post) paralleling her body up against the fence.
The Fence Post exercise teaches your horse that she can move her feet from side to side instead of pulling back when something scares her. When horses get scared they need to move their feet to feel safe. By teaching them to give to pressure and how to move their feet while still being confined, we help them overcome their natural instinct to flee when something scares them.
You can take the Fence Post exercise further and dally wrap the lead rope on the fence (wrapping the lead rope three times on one rung of the fence, then a couple times on a lower rung - the goal is enough tension so the horse will give to the pressure, but if the horse spooks she will be able to untie herself because there are no knots). Now your horse is "tied" and from a safe distance behind your horse you can ask her to move from side to side by applying energetic pressure on the hind quarters or utilizing a flag (horse MUST BE desensitized to flag first before you do this exercise). The goal is the same, the horse learns to move from side to side and parallel her body up to the fence, instead of pulling back when receiving pressure from outside stimuli.
Having a horse that will tie safely anywhere regardless of what is going on around her is a well rounded and safe horse.