THECORBYLOON.
In an attempt to answer your question, it was in all probability because there was going to be a 'three cheers for her Majesty the Queeen'. When this happens, those on parade bearing arms place them on the ground in order to replace their head dress properly, which will be removed and extended into the air during the cheers.
When using the old SLR rifle it was placed between the legs, butt on the floor whilst replacing head dress but this is not possible with the bullpup design of the SA80.