sometimes I think they just need to get the plane to another airport
That is pretty much it. They needed that plane elsewhere otherwise they would cancelled the flight. I was on a NW flight out of Minn, MN once and there were very few passengers. We taxied out on the run way past the de-icing stations - which I thought was odd. Then the pilot came on the intercom and said folks we are having a mechanical problem with the plane. We taxied back to the gate and within a minute or 2 he tells us that we will need to de-plane and make new arrangements. Sure enough we all got booked on another flight 2.5 hours later that was 95% full.