That actually makes a whole lot of sense. Also note, that if Twilight is focusing on her friends, she's not so focused on herself or her magic. It seems that Twilight uses magic best when she just doesn't think about it. Kind of like an adrenaline rush, I guess, where, if she's caught in the moment, magic works better. Like when she accidentally teleported in The Ticket Master. She was sort of thinking about herself, but she was more focusesed on getting away. She even tells Spike, "I didn't even know it was going to happen!"