Actually, this is where it came from. It was a technique developed in Arabic countries. Holding it sideways was actually an exceedingly effective common sense idea that, since we all know when you fire a gun, it recoils up. Well most of the things they shot at were moving or running, so they went "hey, why not hold the gun sideways so the gun just follows them?". Now instead of aiming vertical controlling recoil AND horizontal (to hit their running target) they just had to maintain it up and down and let the gun follow them on its own.
Unfortunately, some idiots thought it looked ballin. The same people also have some idea that "cool" factors into the trajectory of bullets... Wanted was a hilarious influence, i've seen people who thought it really worked... Again, the same people svcked so bad they failed pretty much everything, even recess...
That still only works on 50% of people that are running, because if a right handed person sees someone run rightward, then they either have to switch hands or shoot regularly. Also, you would still hold the gun normally if someone is running directly away from you.
You prompted me to read more on this so other uses are:
1) Targeting large groups of people indiscriminately.
2) Blocking less of a riot shield's view by holding the gun high and horizontal.
3) Preventing shells from hitting lefties in the face.
Those are other semi-logical reasons, it still makes you a bad shot, even with practice, plus you would need to create a specialized sight to get the first-bullet accuracy of a regular gun.