You would have thought so, but many of the people who are the first to stand up rarely get up and walk around during the flight. That's what I've observed anyway, and one of my hobbies is looking out for such oddities... :)
If they didn't get up and walk around during the flight, wouldn't they be the most tired of sitting and therefore the most motivated to stand up first?
There are also any number of other reasons those individuals may not have been motivated enough to stand during the flight itself, as a sibling comment pointed out.
My point was that the original observation was not actually of anything odd but, instead, of something consistent with this particular motivation.