User blog comment:IamJakuhoRaikoben/Chapter 375: Supernatural, Review./@comment-5303512-20140314233709/@comment-7418318-20140314235926

Acnologia also attacked Gildarts while Zeref was peacefully chilling on Tenroujima, so that can't be the only reason for Acnologia to turn up.

Unless Gildarts stumbled too close to the place where Acnologia lives when he's not attracted by Zeref's anger or someone gathering the keys respectively.