In the footage I watched, I saw them exchange a glance, I think they might have planned it beforehand XD Sejeong is quite playful and from one of the behind the scenes it seems Jang Dongyoon is too! One of the staff was checking if he had abs (the scene where Eunho tries to figure out who has an injury on his side) and suddenly he said “Is there any laundry for me to do?” and started scrubbing imaginary laundry on his abs