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Triggered reactions can be intense and unpleasant, and may even overtake our consciousness, as with a flashback experienced by a war veteran. But even more common conditions can have this effect. Think, for example, about the experience of intense nausea. It comes upon a person unbidden, without rational reflection. And you can no more reason your way out of it than you reasoned your way into it. It’s also hard, if not impossible, to engage productively with other matters while you are in the grip of it. You might say that such states temporarily eclipse our rational capacities.

For someone who has experienced major trauma, vivid reminders can serve to induce states of body and mind that are rationally eclipsing in much the same manner. A common symptom of PTSD is panic attacks. Those undergoing these attacks may be flooded with anxiety to the point of struggling to draw breath, and feeling disoriented, dizzy and nauseated. Under conditions such as these, it’s impossible to think straight.

The thought behind trigger warnings isn’t just that these states are highly unpleasant (although they certainly are). It’s that they temporarily render people unable to focus, regardless of their desire or determination to do so. Trigger warnings can work to prevent or counteract this.

As teachers, we can’t foresee every instance of potentially triggering material; some triggers are unpredictable. But others are easy enough to anticipate, specifically, depictions or discussions of the very kinds of experiences that often result in post-traumatic stress and even, for some, a clinical disorder.
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Excerpt from
Why I Use Trigger Warnings
https://www.nytimes.com/2015/09/20/opinion/sunday/why-i-use-trigger-warnings.html