The water system requires some air pressure to work. On the ground this is normally supplied by the APU air bleed. if the APU is not working then this air bleed is not available until engine start, when all the passengers are strapped in and the toilets cannot be used anyway. the issue arises for those passengers that need the toilet on boarding and before pushback. There is also another problem that can also manifest itself in such a way that the aircraft needs some differential pressure to allow the toilets system to work properly. Waste will not leak irrespective of air pressure.