User blog comment:IamJakuhoRaikoben/Chapter 413: The Book of E.N.D, Review./@comment-5037601-20141223205721/@comment-9595671-20141223211122

While we only saw Igneel pop out the other Slayers showed the same signs as him. And it seemed while Igneel stayed to hold back Acnologia the other Dragons knew all the Faces were about to go off so they went to attack them.

Basically, they all exited off-screen so that we wouldn't see them until that moment. It was probably Mashima's way of trying to make it seem like only Igneel was hiding in his slayer at the time. We had no idea on Metalicana's status, Grandeeney had been contacting Porlyusica, and Weisslogia and Skiadrum were thought to be dead. He needed to surprise us somehow.