Our degree of vulnerability to be suddenly triggered from sensory experiences or events we perceive to be threatening to our mental, emotional and physical balance and safety, is directly proportional to the amount of traumatic and unhealed experiences we have had.
This is why it is important to be aware and tuned-in to our own triggers (so that we can help ourselves mitigate them and our reactions to them) and to be aware of what might happen to others, to be able to support them.
For example, a child that loses their friends to an earthquake is at greater risk of being mentally-emotionally triggered by an earthquake drill at their school just days or weeks after their loss.
Similarly, a random conversation with someone about death might trigger in them unprocessed feelings about a great danger they had to their own life (or the life of someone they loved) in the past.