Triggers are something that "remind" a person of a trauma that occurred to an extreme point and causes them to react in very extreme emotional ways - anger or depression usually. Sometimes a trigger is something that when said or done causes a person's MH problem to escalate to an extreme level in a matter of seconds. Triggers are always a very specific thing or type of thing (that never changes)

It is a bad reaction in a person so needs to be controlled and handled quickly.