A long time ago, presumably when the create-a-game function was implemented, Reed decided to give an example sentence for game creation. He chose "A cow jumping over the moon", which is a reference to the well-known poem, "Hey Diddle Diddle".

As a result, there's a huge amount of variations on the topic ranging from hipster cows refusing to jump over the moon, moons jumping over cows and generally mammals jumping over celestial bodies.