I know there are many reasons for an arowana to get drop eye, but I'm thinking trauma, along with genetic traits, are the primary causes of drop eye. If it was due to diet or them looking down, you would expect to see both eyes pointing downwards, but it's more often in one eye than in both. Silver aros are by far the most easily spooked aro (in my experience) and they seem to hurt themselves quite often. While I have nothing to prove it, I would say that maybe silver aros have weak tissue above their eyes (as compared to the other aro species) and after a moderate impact that tissue weakens even more, causing the eye to begin dropping. I'm sure diet plays a role in developing the fatty tissue around the eye, but the initial impact is the primary cause. Maybe the other species of aros have different strength of tissue holding their eyes. The eyes are also quite distinctly different between the Asian and silver species.
This is all just my opinion on the matter anyways. Take it or leave it lol., but maybe it's not so much the arowana looking up in a pond, as it is the aro being able to see the sides of its containment and not constantly hitting it's eyes on the glass when it's spooked that improves/prevents their dropeye.