How does sudden fright or shock lead to a cardiac arrest?

Adrenaline : It's not a very common. But a sudden shock or fright may lead to a surge of adrenaline (your body's way of preparing for "fight or flight"). In someone with pre-existing heart disease or a rare genetic predisposition, the increase in heart rate & blood pressure can result in rupture of a blockage in a coronary artery (heart attack) or directly cause a dangerous rapid arrhythmia (cardiac arrest).

Adrenaline: A sudden fright or other event can cause a rush of adrenaline (aka epinephrine) that then causes an increased risk of a fatal arrhythmia. This would be vanishingly rare without severe preexisting disease.