PTSD diagnosis requires you to meet a set of criteria. A central one is direct or vicarious experience of threatened or actual death, threatened or actual serious injury, or threatened or actual sexual violence. In short, a significant, serious and traumatic stressor. Perhaps as many as 90% of us will experience at least one of these things in our lives, but fewer than 10% of us will go on to develop PTSD. Experiencing trauma is a necessary but, on its own, insufficient criterion for PTSD.https://www.noted.co.nz/health/psychology/trigger-warnings-psychological-problem/
But back to trigger warnings. People with PTSD can be, and are, triggered by things that happen around them. Unfortunately, you can’t assume triggers that are overtly reminiscent of a traumatic experience will be those cues. Warning that a movie includes violence cannot be assumed to trigger violence-related stress, whereas otherwise innocuous things might. You can’t necessarily predict triggers.