On a one way trip with one stop, our first flight was late and the stop so tight that we barely made it onto the connecting flight. We thought for sure the checked bag wouldn’t make it, but halfway on the first flight I got the idea of tracking the bag and noticed it was already loaded onto the connecting flight. It must be a mistake, I thought to myself, but then realized that we had arrived at the airport well in advance and that there was a flight before ours. United probably expedited our bag because our flight was full?