Experience that is already processed can't usually be triggered, or at least not with much intensity: It's not "hot." When stored trauma is triggered we get tricked out of our usual time sense. Past re-animates and hijacks our present.
What gets triggered is core physiology (body memory): a flashback. You can only trigger something that is organized as energetic, persistent, highly charged: available. Unprocessed traumatic experiences are, by definition, dis-integrated, undigested, volitile.
Author Neil Gaiman nicely captures this idea of traumatic reminders or triggers:
There are things that upset us. That’s not it. I’m talking about those images, words, ideas, that drop like trapdoors beneath us, throwing us out of our safe, sane world into a place much more dark and unwelcoming. Our hearts skip a beat; blood drains from our face leaving us pale, gasping and electrified. And what we learn about ourselves in those moments, where the trigger has been squeezed, is this: the past is not dead.
This is different from remembering. This is reliving. The past is not dead. It is not even the past. It insinuates itself as rudely present when triggered. Of interest are trauma-related altered states of consciousness of time, thought, body, and emotion*
A triggered trauma is no mere reflex startle response (jumping if a balloon pops or a car backfires, or you get suddenly surprised). Trauma that gets triggered is state-specific: It can re-present the full flush of the original incident and re-evoke the original event emotions, physiology, mind-set, time, place, state-of-being during the event(s). What was then is now. Stored trauma represents unprocessed experience encapsulated, as if in amber. It will remain volitile until it is somehow processed, dissipated, discharged. Then, it may be possible for the traumatic experience to be integrated into normal autobiographical memory where the event is preserved in memory but without the charge. But right now it is structured as traumatic memory.The drama of stored trauma may very well prevent you from recovering your story and developing and thriving in life.
Traumatic events are part of our biography and part of our biology. Blanking out our history serves a purpose but hobbles us. The goal is to have a choice: One that allows you to remember the old history but live it differently.
Trauma counselling helps people help themselves with this process.
* Healing The Traumatized Self: Consciousness Neuroscience Treatmant, (Frewen, P and Lanius, R., W.W Norton & Company, 2015)