Ditto what AG said. The amygdala learns things through classical conditioning - and that's essentially a permanent learning process. Even if you haven't been exposed to that particular stimulus (aka trigger) for years and years, the stimulus is *charged* with emotional memories. So just one exposure, even years later, can cause "spontaneous recovery" of that stimulus/response process.
Also, the amygdala is more likely to have VERY strong emotional memories for fear. It's the easiest emotion to condition in any living thing, human and alike. And as AG said, your frontal cortex does get flooded by the hormones triggered by the reaction in the amygdala. And the *only* way we know how to control our emotions is by using our frontal cortex. It's the place where we plan everything. What we think, how we act, what we say, deciding whether our reactions to something are reasonable or unreasonable, etc. But when the frontal cortex is overwhelmed and not able to work properly, you just can't temper that reaction from the amygdala. So, if we didn't have our frontal cortex to help control our reactions, we would feel all of our emotions full bore all the time. It's been studied in cats before and the term "sham rage" was created. You could prick the cats very lightly with the end of a pin, and they'd go completely wild because they couldn't control their reactions (their cerebral cortex had been removed - poor things).
But yeah, the process of therapy is all about retraining the amygdala to not react to those triggers anymore. In conditioning terms it's the process of extinction. So, when you expose yourself to something that used to be dangerous (like a sound or smell) but you don't have the negative consequence, you slowly - very, very slowly - are able to learn that that horrible consequence won't be there anymore.
Hope that makes sense.
