User blog comment:BlackWizardZeref/Fairy Tail: Chapter 436 Memoirs Review with a brief mentioning of Chapter 435 Victory Shout!/@comment-67.183.85.153-20150519044811/@comment-24039250-20150519080726

Exactly. I'd bet that Igneel caught wind of Acnologia finding out about END and knew that it wouldn't be safe for Natsu to remain in their current timeline. So Igneel would stay behind and have Natsu sent to Year X77? where Igneel would find and raise him till X777.