I have had them flip upside down in the trailer and never had them hesitate to go back in afterwards. You name the fear, and the reaction to it and chances are I have had it happen. I cannot recall one of those horses being adversely affected by it for any great length of time. By the same token, I have had horses that have performed an action flawlessly for years suddenly become afraid of it. Fear can be caused by a change in the situation, pain, a part missing from a sequence, a part added to a sequence, too many people around, not enough people around and what have you.
Fear can pop up at any moment.
The horse looks to the herd leader for justification of the fear and the reaction to it. If the herd leader is acknowledging the fear THAT acknowledgement gives it credibility. In that case, instinct says it's better to be safe than sorry and fleeing is the only logical option. Some horses are just born more nervous than others. Before my concepts became instinctive to me, I did the best I could with that nervousness and over time, it became manageable.