Pulling back when tied is a common, and dangerous, horse behavior problem.
Horses that pull back when tied generally fall into two camps:
most commonly seen, are horses who experience genuine fear and panic when they feel restriction from the halter
much less commonly seen, are horses who don't appear to be fearful*, who pull on a tied leadrope until it breaks
No matter which camp a horse falls into, pulling back when tied can be a dangerous behavior - for both people and horses. When a horse feels the restriction, they panic and pull back, struggling violently to escape. While this happens, the horse is unaware of what is happening around them, and may easily injure themselves or anyone in the vicinity. Therefore, this is a problem behavior worth trying to resolve.
Origins of the behavior
Usually, horses who pull back when tied have not initially been trained to yield to pressure and restriction from the halter. Others may have been initially trained, but then had a negative experience when tied which resulted in them panicking, and fighting to escape the situation. Either way, resolving the issue is best done using a two-pronged approach:
Managing the problem, in both the short and long term
Using effective, low-stress techniques that have little risk for making the problem worse, or injuring the horse
In the short term, management involves not putting the horse in situations where they may pull back. This means not tying them, period, until retraining is complete. Because restraint is part of most horse's daily lives, in the short-term in can be helpful to teach the horse to ground tie or station using positive reinforcement training methods. Alternatively, the horse can be confined to small areas such as a stall for grooming, saddling etc.
Such management approaches make it impossible for the horse to pull back when tied. This helps keep people and horses safe, and also prevents the horse from become 'practiced' at the behavior of pulling back. Behaviors that are repeated are more likely to occur in the future, and the longer they occur the harder they are to stop.
Retraining the horse who pulls back when tied
Retraining the horse who pulls back when tied is possible. Because fear is an element of this behavior, this is best done by using two techniques used by horse behavior consultants to solve even the most serious fear-based issues - systematic desensitization and counter-conditioning. Systematic desensitization (SD) involves gradually exposing an animal to the thing which it fears, in small enough levels of exposure that fear is not triggered. Counter-conditioning (CC) means changing an animal's emotional response to a thing or situation - from a negative emotion such as fear, to a positive one. Emotions occur involuntarily, and powerfully affect how an animal behaves. When training problems involve negative emotions such as fear it is critical to use techniques such as SD/CC to address the fear. For the horse that pulls back when tied, this would involve gradually exposing the horse to all of the steps that result in being tied, culminating in being tied, all the while pairing the experience with something pleasant - like food or scratches. Most horses also benefit from additional training to learn how to step forward in response to pressure from the halter. This is best done through the use of shaping - rewarding progressively closer attempts to the end goal. Using a light pressure on the leadrope is best, coupled with providing something pleasant the moment the horse responds to the light pressure.
It is crucial to avoid triggering fear or pulling back behavior during this process. Fear is counter-productive to learning, and can quickly escalate the horse's pulling back behavior. If this were to occur it's not the end of the world, but one would need to start over again and the retraining process would take longer each time it happened. Triggering fear or pulling back behavior is also completely unnecessary to address this problem. I have successfully retrained numerous horses that pull back when tied using this approach, without once triggering fear or causing them to pull back. When the behavior has been occurring for a very long time or when the owner is unable or unwilling to attempt retraining it may be safer to permanently use other methods of restraint, such as teaching the horse to reliably ground-tie or station on a mat, or using portable panels at shows etc.
Long-term management of the behavior
Behaviors that are learned in fear, such as pulling back when tied, are prone to relapse. It's important to understand that this is not a failing of the retraining program, it is simply a fact of how the brain works. Brains are hard-wired to remember lessons learned in fear, and given the right conditions the fear about being tied can return. Once retraining has been successfully completed it is still generally advised to use a tying device, such as a Blocker Tie ring, to tie the horse. Such devices allow the leadrope to 'slip' a little if the horse pulls back, minimizing both the horse's fear and the chances of relapse.
Methods NOT recommended to retrain the horse who pulls back when tied
Suggestions on how to resolve pulling back are easily found on the internet. Unfortunately, most compromise the horse's welfare, may be dangerous to put into action, and can worsen the problem or even create new problems. As your horse's champion, it pays to be informed about the risks that accompany these suggestions.
Flooding is a procedure that involves exposing an animal to the thing that it fears, at full intensity, until they stop reacting. For horses that pull back when tied, this may involve tying the horse 'hard', so that they cannot break free, and then allowing them to fight against the restraint until they quit. This approach frequently results in serious injury or even death to horses, and also puts people working with the horse at serious risk. Trying to control an 1100 lb panicking animal is extremely difficult, if not impossible; if a horse manages to break free during flooding the behavior of pulling back becomes even stronger, and more resistant to change. During flooding, a horse may also develop 'capture myopathy', life-threatening muscle damage that results from struggling violently. Flooding is a serious health and welfare issue, and is also completely unnecessary to successfully resolve the problem.
Flooding can occur as described above, or it may involve the use of other paraphernalia. Examples: ropes around the neck, legs or body to exert pressure in other areas when the horse pulls back, or even while using a device such as a Blocker Tie ring. Regardless, flooding is occurring if the horse is tied without opportunity to escape and the pulling back behavior is triggered. Flooding also occurs when a trainer deliberately creates a situation that causes the horse to pull back, such as by waving a flag etc. No matter the set up, if the horse panics and pulls back during training it is being flooded, and horse owners should avoid such approaches.
As your horse's owner, you are their champion. Educating yourself about horse behavior problems - like pulling back - will allow you to choose effective training techniques that also promote good horse welfare. If your horse pulls back when tied and you are not comfortable addressing the issue on your own, seek help from a trainer or behavior consultant who uses a combination of management, counter-conditioning, systematic desensitization, and shaping to resolve the issue.
*It's important to understand that behaviors such as pulling back are created as a result of a fearful horse panicking to escape. In a very small number of cases, fear can appear to be absent when the horse pulls back. This occurs because repeated, successful escapes (breaking free of restraint) naturally result in a decrease of fear. But, the fear memory is never erased from the brain, and given the right conditions it can be quickly reinstated. In light of this, the same approaches as discussed above should be used on horses who seemingly show no fear when pulling back, to avoid reinstating the fear that accompanies the behavior.