I would say wait for some news about them having technical difficulties, but it really doesn't look like it was and they just got cut off.
I think if they did have technical difficulties, they would have been allowed to perform again, but like has been said, they were shooed off the stage and weren't allowed to finish their performance and come back on the stage.
If this is true, then this is so disrespectful! Not only to Apink themselves but also the fans who waited for them to perform!
I hope KBS has a very good reason why they were cut off because that is no way to treat any group/artist, let alone senior artists!