Did you read all the "Vader's tiny hands" comments that littered the net after the preview trailer debuted? I'm guessing that was a huge part of it.
But then I think about the Deluxe Wave 2 toy, where his hands are down, which makes me wonder if there is something more to it. I mean, those have much longer lead times, so there had to be reference of the hands down more than a year ago.
Maybe there was more to the scene between where the original "hands-up" shot was and the later "hands-down" shot? Something justifying the arms changing position, and when that was removed, they altered the "hands-up" shot to help continuity?