User blog comment:FairyRaven/Fairy Tail Chapter 400: Wings of Hope, Review and Commentary/@comment-1359322-20140913124543/@comment-7418318-20140913170241

Maybe Acnologia just wants to kill E.N.D. because he hates him and Acno just didn't manage to track him down before now. And that's only if Mard's theory is true, but even he initially said that he didn't have an idea what was going on.

It's also possible that Acnologia is worried about the Faces and thinks that taking out the base of the Demons might be enough to stop the activation, or that it was drawn by the malice or magical energies that were used there or something else we don't know about yet.

I just thought of a possible other theory. At no point did Natsu get as close to dying as Tenrou (against Zancrow) and just now due to the poison. Maybe Igneel fed a bit of power to Natsu in these instances and Acnologia felt that and came to hunt down Igneel. But when he couldn't find him, he took his frustration out on the Fairy Tail mages instead.

But then it stands to question why he didn't react to the time-travelling Dragons... hmm... Maybe the Eclipse event interfered with his ability to sense them?