The water you see just before start up is running out of the aircon pack ram air inlets when the packs are shut down for engine start:
1 - When the packs are in operation, water is separated out of the bleed air passing through them, & is then sprayed into the ram air inlet duct, where it cools the primary heat exchanger, thereby increasing its effect.
2 - When the engines are started, the aircon packs are switched off so that bleed air (from the APU) is sufficient to crank the engine over.
3 - With the pack(s) off, there is no longer any airflow through the ram air duct & water runs out of the front of it under gravity.
Often reported as a fuel leak by new pushback crews.