If there is no pain/ulcer component it sounds like a confidence issue. When startled she completely shuts down and goes into defensive response mode because she’s low in confidence and genuinely afraid. Building her confidence and teaching her how to process even when scared should be a huge help.
A friend of mine has a very low confidence reactive mare. When startled she goes from 0-10 in the blink of an eye and will throw in a ton of buck, spin, take off cantering crap. For years she was a sticky ride and rode it out. She’s now getting to a place where she is able to command her mares attention more so outside stimuli is less important. The mare may still startle but she’s so focused on the work that she knows exactly where the next foot is supposed to go.
The TRT method is a really cool philosophy on how to build confidence so that the horse feels less of a need to have an explosive response to a scary stimuli.