Hold on a moment. Let's cast our minds back to last year and that early leak of pompeii-esque still image...
In that scene on the steps, human-like shapes are seen battling what looks like Xenos. So wouldn't that mean Xenos were already on the planet BEFORE the arrival of the engineer ship and release of urn-bombs?
Otherwise it would seem a little too instantaneous for the release of the goo bombs to kickstart an evolution of xenos, and moments later wipe evrerything out, including the petrified remains of the xenos.
And in any case... How come the planet civilization has no sophisticated air traffic control system or ground to air defence capabilities? Surely it can't be as simple as David rocking up to their planet uninvited in order to trojan horse the heck out of everything.(Yes, its possible David could master their language and flute, but to a foolproof level?)
Unless... The engineers are so busy battling the Xenos, they don't have time to manually intercept the arriving ship. So the scorpion tail manages to finally go into auto-defence mode after the release of the goo bombs, eventually flnging the engineer ship aside to send it crashing and embedding itself precariously into the side of a hill.
Assuming the scorpion tail is a docking device, there has to be more than just last minute dock and go. What about comms systems and foolproof handshakes designed to intercept objects before they even reach planetary orbit, never mind the surface?
I'm just having a last-minute panic that the movie pulls a Prometheus - lots of breathtaking imagery and design, but beneath the surface very little makes sense.