First you need to work with the horse in the barn in the crosstie area to give to pressure. Give to left and right pressure and forward and backward pressure. The horse should not be tied until this is mastered and giving to pressure is immediate. When re training a spooky horse on the ties I use two ropes attached to bailing twine so they break easy. then when they break horse is returned to the ties immediately. You should have a back up pair on standby so you can fix the ones that just broke. The problem could be how you reacted to the how they broke the first time. If the horse got any type of reward like not having to stand back on ties immediately he or she may have just been trained to break them. Also the horse should be worked regularly and not have excessive energy that would cause him her to be on guard.