The Juggernaut was programmed to attack automatically?

That would be a big ooops moment for David! He would do like he did in Prometheus when he cracked the code to the door....."Oops...Sorry." lol

It's not a bad theory in itself TC, but David has probably read most Engineer tech instruction manuals, ranging from advanced flute magic to squishy button-pressing guides. He knows what he's doing.

You do not rip the head off an android, and use it as a weapon to hit someone else. It's bad for business. 

Interesting to speculate, but i think if the Engineers had this kind of Technology then it would be set up as part of the Navigation system.

And so i dont think they would have it Target Automatically to Drop the Payload on every world they decide to set their Sat Nav for...

David would surely know this would be the case, and i would assume there would be a command that would set a course to a World and not a auto-deploy.

I think its not coincidence that David is watching from where the Urns are being dropped. And so it is very very likely this was his intention.

It depends on the timing though, does David hover the ship and then does he casually walk to the Cargo Bay and time it just right before the floor opens and the Bombardment begins?

Can he operate the Weapon from the Cargo Hold?  Where is Shaw at this point?

So many questions but we can be certain David intended this.