I had a similar experience. My flight from YYZ-EWR was delayed. The previous was delayed and came back to the gate for re-fueling. Then, a few pessengers were put on the previous flight, so that they won't miss their connections in EWR. But, unfortunately, only the pessengers without checked bags were allowed to get on the previous flight.
It seems to me, this is a quite reasonable solution. The previous is delayed anyways. Why don't take a few more if it could?