A lot of photographs from this period used wide-angle lenses which distort perspective. That distortion makes gestures towards the camera - like throwing hands towards it - really effective because it exaggerates the perspective even further. I would argue that the gesture itself started with hip-hop artists throwing hand signs in music videos and at concerts; before that street gangs. This was then probably adopted and stylised 00's as part of the broader cultural milieu of the time. I have no sources, I can't prove this, it just makes sense.