Fuel and Oxidiser tend to slosh around when subject to loads (i.e. re-entry), there will be design loads on the tank structure for how much propellant can remain in the tanks without damaging the ship on atmospheric re-entry.
They would need to dump to prevent the ship becoming damaged and potentially breaking apart at this point.
You would dump with the engines running as the propellent would separate and move away from the vent port and also hover around the ship until re-entry.
Given on the first launch they had a number of engine failures, it's logical that they would add more propellant to give a greater chance of reaching the target trajectory in these cases.
Also given there was no payload the extra propellant mass will help to simulate one, otherwise the forces experienced during launch will be higher and not a representative test.