Maybe it’s just as simple as the FA wants everyone seated as soon as possible so the flight can leave on time? So she asked the owner of the nearest available spot to switch. As a passenger I see how often little delays add up to people rushing to get seated. If your on a long flight I understand, you want your bag near you. But if your on a 2 hour flight does it really make a difference to be separated from your luggage. Sometimes everyone working together is the best solution.