User blog comment:IamJakuhoRaikoben/Chapter 375: Supernatural, Review./@comment-5303512-20140314233709/@comment-24066070-20140315064851

GH had the keys before they set sail for Tenroujima, so if the keys attract Acnologia he would've intercepted them before they got to the island. Then again he likely had to cover quite some distance, since it took him many chapters to reach the island.

What really attracted him was Zeref's power I think. It was Zeref who turned him into a Dragon and perhaps he wants to get revenge on him, though he did ignore the airship and happily trashed about on the island.

Zeref used his death bursts quite a lot and Precht turned some pebbles into Demons by using Zeref's spell. So Zeref's magic may have been sensed by Acnologia and all this magic was used due to GH, therefore Zeref blamed them for summoning Acnologia.