OK, that knocks out a few theories, since that is just a A320-232, 20 years old. There was a bit of an issue when BA starting cramming in more ET seats on the newer aircraft that there was a risk of loss of balance. Delta Kilo is loose loaded. I will await the arrival of the Flying Corps but my sofa version is that with the burn off of fuel, there was something quite heavy on board that could perhaps cause problems and therefore in an abundance of caution the cabin crew's requirement to wait before disembarkation.