Maybe the ones who were sitting had just stood crammed into the carriages of a train for 3 hours from Rugby to Euston and needed to sit down!! :-)
I was on the lower tier and the stewards came along a few times and sort of gestured for people to sit down, nobody took any notice but to be honest sitting in those seats would have contravened health and safety anyway I am sure. There wasn't room to sit properly because your knees were squashed against the seat in front. (I sat at half time having been one of the ones standing for said 3 hours!!)
The stewards on the lower tier were friendly and just let us get on with it. Only thing was that them standing in the aisles obstructed the view for a lot of fans anyway. A little lad across from me missed the first 4 of our goals because of them, his dad had asked specially for an aisle seat for him which is not much good if the stewards then get in the way!!