Maybe trying to cover up that they forgot to fill up the tank before leaving the gate. If on a road trip I realize that I forgot, I’d prefer to say it might be good to top off the tank before we go any further.
To be honest, that was my first thought. Somebody screwed up.
Maybe while they were taxiing ATC changed their route, or winds aloft changed, necessitating additional fuel. I think they usually plan their fuel loads pretty precisely.
“...fill up...”
Planes usually carry enough fuel for the flight plus reserves. They don’t fill up; fuel is heavy and is costly.
If the pilot taxis too long or finds out there are greater head winds on the flight after gate departure, he would then return to get more fuel.
He was doing the right thing.