What are 'triggers' – automatic responces

What does a therapist mean when he/she talks about triggers… when they say that my son was triggered by something?
Quick answer – a trigger is anything in the environment (person, place, thing, thought etc) that causes a predictable reaction in a person. Very often people are unaware of their triggers… they might be fully aware of the reaction… but they are not always aware of why they seemingly reacted automatically and without intention. Triggers very often cause an emotional reaction that does not fit the current situation.
As in the literal case of a gun (from which the metaphor was probably taken from) if you pull a trigger… very predictably, a bullet will shoot out. 
Some of you might remember the study in which the scientist trained the dog to salivate every time that he rang a bell (the bell is then the trigger and the salivation is the automatic and unconscious response)… the very same thing can happen in humans. 
the reason that therapists talk so often about triggers in the context of relational or behavioral difficulties is to help people to understand that one, often people are reacting automatically and they truly did not ‘think before they acted’, and two, often a ‘triggered person’ will have a reaction that seems to be out of place given the current circumstance.
For example: lets say there is a guy named Bob who seems to always get ‘excessively angry’ any time that someone enters into a room without knocking first. Bob has a wife named Suzie and they are currently seeking counseling as Suzie feels that she is being unjustly treated as Bob yells at her for “small things like not knocking before she enters her own house.” When Bob was 12 he was in the bathroom using the toilet and his sister and her friend walked right in as they forgot to knock. The girls all laughed at Bob and he was incredibly embarrassed (note: it is possible that Bob remembers this story very well and it is possible that he only vaguely remembers this occurrence). Bob believes that he is justified at getting angry at his wife when in truth his emotions are mostly the result of being ‘triggered’ as he has an automatic emotional reaction to people entering rooms without notification. Therapy can help people to uncover their triggers so as to avoid acting automatically as Bob did.
Why do we have triggers then? Triggers are very important to survival. If we did not react automatically to certain stimuli we would never be able to consciously make choices in time to avoid potentially dangerous situations.