Tax reform!
Damn fuel taxes.
It might have been someone goofed on the fuel calculation, and the error was discovered while the plane was taxiing. Or it could be the forecast weather at the destination changed and the new weather increased the reserve requirement. If it was an afternoon flight, the taxi delay might have changed a daylight landing to a nighttime landing, which has an increased reserve requirement.
They reason they call it “payload” is that if you don’t pay, they won’t load it. Fuel included.