Similar to "Fives" or calling "shotgun", "Quack Quack Seatback" is a common way to lay short-term claim to a seat you're about to vacate when with a larger group. Sometimes shortened to "Quacks."
Person A: "Hey, you stole my seat."
Person B: "It's my seat now."
Person C: "No, he called Quack Quack Seatback. Move it."
Raison de quack quack is inspired by the raison d'etre ... For ducks that would be, to be is to quack. For humans, to be is to ... quack like a duck? Or something else? (1) Being horny and (2) in the mood for a good time.