There are normal pacemakers and then there are ICDs or Implantable Cardio Defibrillators. Pacemakers are used to pace, logically, patients who would have a very slow heart rate normally.

ICDs also pace but, are constantly on watch for irregular heart rhythms that could be fatal. You will often see the portable defibrillators in public spaces. Some of us are at enough risk that we carry our own.

When mine delivers "therapy", they are loath to call it a shock, it feels like I have grabbed an electric fence with both hands. It will take me to my knees and you will hear some kind of audible from me since about every muscle in my chest just contracted.

The passenger may have received a legitimate therapy or anxiety could have drove their heart rate above the trigger threshold.