A question about the opening scene

Welcome aboard...

I think we have to remember its Sci-Fi and some things just look Cool for Aesthetic Reasons alone, and so i dont think any Thought was given to WHY the Ship appeared to Change its Orientation.

It seems it was Horizontal to be able to Drop Off the Engineer and then it went Vertical in order to attain a Escape Velocity to leave the Planets Atmosphere

Which is what we see with the Space Shuttle, as it is sent into Space Vertically via Rockets, so i can only ASSUME the Engineers Ship has a Propulsion that allows it to Escape a Planets Atmosphere etc where this Propulsion is at the Rear of the Ship that would be Facing Towards the Ground when the Ship moves to the Vertical Position.

By aligning itself vertically, the ship has less air to push out the way if it wants to fly straight up. If it can hover silently, it can probably move in any direction it cares to, but no matter how advanced its propulsion system is, aerodynamics is a consideration when you’re in an atmosphere.

it may well be the case that the ship has a high power mode (as Big Dave suggests) with the main propulsion at the rear. That seems to be the case with the Juggernaut that leaves at the end of the movie. In that case it was heading off at a different elevation, but still minimising the amount of air that it displaced.