Actually the holding the gun sideways was a practicality thing. When unloading a magazine rapidly you have limited control of the weapon's recoil. When holding a gun upright the recoil moves it upwards, holding it sideways moves it sideways.

Now imagine you're a gangter, 'bout to come up on some punk steppin' on your turf. You an da boyz gatted up ready to throw down. Get in the low-rider with your illegal Tek-9. Roll up on those fools ready to shed lead. Which way do you want your recoil going? Upwards? Or sideways.

Rat-tat-tat.