The max takeoff weight is determined by the most limiting of about a dozen different factors, and the fuel loaded onto the plane is determined such that you are at your planned takeoff weight when you reach the end of the runway.
But sometimes things change after you push back from the gate (in this thread's first poster's case, they went to a shorter runway than planned) and you have to get rid of some of that weight, which you wouldn't have loaded onto the plane in the first place had you known how things were gonna go.
Edit: oops, I just re-read the first post and that's not what happened. But it just as easily could have.
Reminds me of the time I had to burn off the most fuel... In a short haul airplane on a short flight, MTOW was that flight was limited by max landing weight at the destination. Due to a rerouting for storm passage that drastically cut the flight plan distance, we had to lose 2000 pounds... which was a quarter of our fuel load! I wonder what the record is, percentage-wise. Maybe we hold it?
Last edited by Vessbot; 21st Aug 2017 at 00:43.