The BA flight was oversold, which is why your originally booked seats were not available, and you got an upgrade. The easiest solution for the BA gate attendants was to offload all passengers connecting from the late LAS flight, which is what they did! I guess they were lucky only 10 people actually got to the gate, as they probably had enough seats available.
The ECJ ruled if you get to the gate during boarding, but your seat has been given away then you are due the full compensation.