If you saw say a dramatic car accident, your body would naturally react by focusing on it. Your eyes widen. You become so hyper focused that time seems to slow down. This instinct probably helped our ancestors better avoid perilous events by being sure we took in every detail when they happen so we could better avoid them in the future.

When you are on the edge of a cliff and think about what would happen if you fall, your mind identifies that as another perilous event. It starts to become hyper focused on that thought just like seeing an actual perilous event would because it has trouble distinguishing the difference. It's not that you want to fall off the edge. Quite the opposite. But you can't help but think about falling off the edge because your instincts compel you to become focused on such events.