Not uncommon to limit number of passengers on a plane and/or shuffle them around to balance the plane out, but this is more common on smaller regional planes. Certainly after the door is closed to the plane, and everyone is seated, is probably not too common.
The weight count (passengers+cargo) should be performed earlier I think. But, maybe a late change in flight path and/or unexpected weather enroute might require extra fuel and the need to offload some folks? Just some thoughts