It does happen where a calm horse will get too scared and seemingly lose his training.
John Lyons horse Zip had the same thing happen to him. He will tell the story of a photo shoot they did involving a helicopter, bear, and a tarp that shot up into the air. During the shoot Zip was fine, but the next day he was really spooky. John had to take him back to the round pen and work on spook work.
I too have had this experienced with a Mustang and a PMU rescue mare. In both cases the horses were 'fine' until one day the horse jumped and the rider came off. Both horses were fearful after that day and had to go through retraining to get them thinking again. Both horses were also very fearful of the area where the accident took place.
My suggestion is that you go through spook work with your horse and reteach the horse how to control it's fear. BTW both of the horses are doing super on trail and the owner of the mustang says he is so calm now that he goes a bit too slow. : )