Once a person goes through the experience of having his or her amygdalae set off, a number of things happen. The person almost immediately goes into either ‘fight’, ‘flight’ or ‘freeze’ to deal with the threat. These responses are NOT voluntary. Your body’s survival mechanism has to assess and decide which response will best ensure survival.
Your senses record all of the sensory data: the quality of light, certain sounds, textures, smells, etc. while the event is occurring. This is to ensure that the next time one or more of these sensory situations occurs, that the amygdalae will ‘smell smoke’ and set off the alarms immediately in order to give you the best chance of survival.
These sensory details, when experienced in an otherwise non-threatening environment can send you into fight/flight/freeze as the reptilian brain is not rational and cannot intellectually work out that though the sensory stimuli relate to the original trauma, this is not a threatening situation.