It's unusual for trousers to be neatly laid on a dirty kitchen floor for no purpose, so maybe they were put there to prevent the spread of blood from Nevill's head wounds, which was running down the side of the coal scuttle. Ditto for the three cushions and towel nearby. Why would the assailant do that if he wanted to be out of the house asap? There seems no logical reason, unless it was to prevent transfer of blood from his shoe soles should he step in the pool, if he intended traipsing through the house again. Would the police/TFU interfere with the crime scene by doing the same?
The trousers might have been airing on the ceiling drying rack above and just in front of the AGA, so were conveniently to hand to lay on the floor.
What is more interesting to me is the fact that Nevill was perched astride a chairback in the most uncomfortable looking position of any murder victim I've seen. Did he end up there on his own, or was he lifted there by the assailant? Some even say he was moved there by the TFU who barged in through the kitchen door, pushing Nevill over who was seated just behind it.
I think his pyjama bottoms simply dropped down during the struggle, which may have caused him to lose balance and end up where he did.