Question about strange United departure today 5 April

[AdamB]

I was at the beach this morning & saw what was to me an odd event. A United Airways 744 (flight number UA839, reg N107UA) taxied parrallel to 34L tailed closely by a fire truck & a ladder truck. It sat at the end of the taxiway for at least ten minutes, then rolled onto the runway & very slowly made its way to the taxiway for 16L/34R where it stopped. My scanner was a bit scratchy but I did pick up the tower saying "it's up to you if you want to proceed or return to the apron". It sat on the taxiway for half an hour then taxied down to 34L for the shortest & fastest rotation I've ever seen out of a 747. Flightstats shows this flight as 34 minutes late into Melbourne. Anyone know what was the cause of the delay or can enlighten me as to what I was watching unfold?