Something has definitely scared them. Just like people ducks respond to fear differently each time. Think of it this way, someone makes you feel uneasy, but you only see them every once in awhile so while scared you're not panicking. Then one day you see this person peaking in your window and you lose it completely panicking. When ducks see predators at night it's like seeing Jason with a Machete standing out the window. The predators want to eat the ducks and the ducks are 100% aware that they are the prey.
