[At first, Rei is just confused. She hasn't had a robot ordering her around since she had her chain on - did she do something wrong? Is she being punished?
Then, as she walks outside and sees everyone else, she finally understands what the music and GIR's voice mean. It's some strange sort of dance party, which everyone appears to be forced into doing. She stands still, but one of the robots prods her painfully, making the truth very clear - she has no choice but to dance.
And since she has no idea how to dance, she'll just be rocking from side to side, occasionally switching from one foot to the other. Also looking around her awkwardly - so many people. More people than she likes to have around at one time.]