If you wtch again closely, the problem is that she has the watermelon sitting up to high in the pouch. When she released it, it tried to center itself, but ended up flipping over because there was too much movement for the melon to get straight/centered.

We used to shoot water skiiers with water balloons from about 300 yards. My best shot was hitting a skiiers ski, which sunk it and the skiier went face first into the water. We were on a deck up on a hillside. Poor bastages never knew what hit 'em. The timing was the most fun to watch because it took about 3 or 4 full seconds before impacting with the water.