Once Upon A Time, there were two Bad Witches. They happened to look a lot like the King and Shark Boy, in fact, it was speculated that the witches were merely that Father and Son on a game of magical make-believe, but that is not important at the moment.
What is important, is that they only had one broom between the two of them. Bad Witch the Littler invited Bad Witch the Bigger to ride along, but when the Bigger tried to straddle the broom from behind, it proved difficult to stay on and keep up while flying, at least not without possibly kicking or knocking Bad Witch the Littler down.
The only solution was to ride side-saddle with both legs on the same side of the broom; after all, being a Witch is no excuse to not be a Lady.