Not completely clear on what's going on in this video but one of the girls (think her name is Gyuri) has a sign on her head which says to tickle her so whenever she tries to sing, someone tickles her until she collapses to the floor. Starts at 0:45.
https://m.youtube.com/watch?v=sRODrlwoW_E
https://m.youtube.com/watch?v=sRODrlwoW_E