What's happening is that upgrades, etc have been cleared so some seats that were occupied are now free. People are then seeing those seats are available, and moving into them, freeing other seats.

Think of it as if you are standing on a bus. Someone gets off and a seat becomes available, so someone moves and sits in it. Then a better seat is open, so someone else goes and sits in it.

Then, a few people get off at the next stop and there are 6 or 7 seats free that are all together near the door. People who were sitting at the back rush up and take those seats, which lets some of the standing passengers sit down at the back.