How could this happen?
The a/c had to be towed up to park. The marshaller was giving signals to set brakes. The pilot made a boo boo.
It looks like it starts to roll with more than idle power.
On top of the boo boo there are two things I don’t get. 1, are the engines still running? or 2, if the engines are shut down does the ramp have enough slope to let a 747 roll on its own?
Good thing the grounds crew got out of the way!
The ground crew looks like me in a pen with a mad cow! 
All very good points. The engines to the best of my knowledge would need to be shut down to tow in. ALSO, you are right why and how did the throttle manage to be high enough to roll and push that tug?