That's the easier thing to operate since the aisles on the 787-9 are fairly narrow and if someone is already in the aisle getting their bags out of the overhead lockers, someone else in CW already ready to leave and away from their seating area, it's sometimes just easier to let the small number of CW passengers off the aircraft first to allow clear passage to the First passengers. The cabin crew then stand at the end of the aisle at 2L so that passengers in the second CW area cannot leave. What the OP saw was the far more tricky job of keeping people in that front CW section in situ until First had left.