Most common reason a shot is flipped is to avoid breaking the 180 degree rule.
This is never the fault of the editor, as they have to work with the footage they have in order to make a scene work shot-to-shot. Breaking the 180 degree rule is far worse for flow of a scene than a reversed image, hence why they tend to pop up a lot in many movies. We only notice them more in Star Wars because we observe them repeatedly, much more than any other film.