Once a trauma is encoded in your memory the brain can be triggered by something you see, smell or hear. The brain will pull the past memory not realizing that you are not there in that situation. Then the body will activate your flight or fight response thus why you may have such an intense response to the memory.
Conscious memories of trauma are encoded by various sites in the brain which process different aspects of experience. Explicit memories of trauma reflect the terror of the original experience and may be less organized than memories acquired under less stressful conditions. Typically they’re more vivid, more intense and more persistent.