Was it a very small plane with weight and balance issues? Did the FAs want the seat for crew rest or to upgrade a buddy into the better seat?

Otherwise, I've always felt free to just move quickly without asking to any vacant seat in my cabin after the aircraft door is closed. I ask if the plane has apparently boarded and we're sitting at the gate or sometimes on long haul I'll ask the purser early how full BE is expecte to be if I think I may try to move.