Why do riders swing the bike out sideways when jumping?
It's a trick -- it looks good on camera and would count for points accordant with degree-of-difficulty and "sticking the landing" at an adjudicated comp. The bike and rider become effectively parallel with their upright-riding stance; as opposed to perpendicular when in contact with the ground.

It's also a marketing cliche in biking. Like an "ollie" or a "kick-flip" on a skateboard - landing this grants credibility and implies talent, courage, maybe stupidity ... depends on the actual vs. perceived talent level. 

To be clear - cuz looks cool.