Why did the loon lower its head over the nest?

A loon will lower its head and neck over the nest when it feels threatened.  Yesterday late in the day a viewer noticed the loon was in this position because a boat was in the area. This position indicates the loon may flush from the
nest and leave the eggs to overheat, chill, or be taken by a predator. If you come across a loon in this position, please back away quickly!