User blog comment:FairyRaven/Fairy Tail Chapter 400: Wings of Hope, Review and Commentary/@comment-125.60.173.148-20140913130408/@comment-7418318-20140913150453

If it was Acno's mere presence, it would have happened on Tenrou already.

But considering that last chapter Igneel wondered if it was time to come out, maybe he opened some kind of seal that hid their presence.

Or maybe it was just the first phase of whatever process let's the Dragons come out of the DS's bodies again.