وَقَبَ (when it settles): its darkness settles on everything. Sometimes the evils are unseen by man so he cannot avoid them such as a wild beast, an awaiting enemy or a plot or conspiracy in night. All of these become clear by day. So the word (settles) indicates that it is a must and we have no power to avoid it.
The origin of the Arabic word وَقَبَ (settles) is a hole in the mountain resulted from the heavy continuous rain.