Situations in daily life can act as reminders of unresolved, traumatic events from the past. When that happens the person will get "triggered."
When a person gets triggered, all the unresolved thought and emotional energies that are still trapped in past events will start bubbling up into conscious awareness. When this happens, the person will relive the event all over again.
That can bring on a panic attack.
When a person gets triggered it can seem like it's coming out of the blue. As a result, the person feels out of control.
You can't talk a person out of a panic attack. You need to find out what's causing it and resolve that.