Well, left to right is the way we read so in a way right-to-left is viewed as retreating, or at least I was taught that in a graphic design class in colege.

IIRC the LOTR films almost always has the heroes cross frame left to right in all the films.I don't know about 'natural vs. unnatural' but it kept a sense of consistency to the imaginary geography the audience had to construct.

As for AOTC, it is true that in the OT the heroes went left to right (think Hoth) and Lucas flipped that in AOTC, perhaps for no good reason, perhaps to emphasize that the Clone Army was the start of the Empire.