Think about how imprinting works. At 1 - 5 days old one of these adorable fuzzballs decides that you're Mom. He follows you around (making that cuteness overdose inducing little pitter-patter with those little webbed feet on a hardwood floor), tweets in distress if you're out of obvious range for more than a minute or two and tries to bury/embed itself on in or under your body.
And ten years later, that duck will still recognize you as Mom.
Second, ducks are prey animals. Their main defense involving spotting trouble at a distance and then swimming/flying away from it. If ducklings are involved, a mother duck relies on aggressive charging or other displays to bluff predators into leaving her and her babies alone.
Ducklings aren't even prey animals. They're like the snack food of nature. Of a flock of ten hatchlings in the wild, a mallard mom might see one of two make it adulthood/flight capability.
So, given all that, if a duck gets into a bad situation (especially as a duckling) and somehow survives, the duck remembers because it knows very well that Mother Nature never hands out third chances and second ones are very few and very far between.
