User blog comment:FairyRaven/Fairy Tail Chapter 414: Drops of Fire, Review and Commentary/@comment-5006572-20150105153905/@comment-7418318-20150105154319

The first battle with Acnologia was very short. Maybe coming out is a process that takes time and the Dragons didn't have enough time to get out of the Slayers back on Tenrou.

That argument would of course only work if in the Tartarus arc the Dragons had already started the process some time before Acno turned up, maybe due to Face being such a big threat.

Or they were caught off guard by Acnologia's first appearance, but anticipated the second one.