My head canon for the following is that Twilight has been possessed by King Sombra.
T: Look at what I've got here... *Rainbow Dash struggles to break free of Twilight's magical hold* RD: Twilight! Snap out of it! Sombra is using you! T: No Dash, I feel more free tha I've ever been. Now then, I've always wanted to see a star separation... *Twilight then levels out Rainbow and holds her in place* RD: What are you doing?! T: I'll be sure to say hi to the others for you... *Twilight begins to slowly pull Rainbow's body and limbs taught, causing absolute agony for Rainbow* RD: *Screams in pain* OH CELESTIA!! PLEASE! STOP IT!! IM BEGGING YOU!!!