User blog comment:FairyRaven/Fairy Tail Chapter 346: Retrogression Magic, Review and Commentary/@comment-7418318-20130802133028/@comment-7418318-20130807150354

I was wrong it seems, there was a reason after all why Minato can enter this Kyuubi mode, even though it's a bit sketchy if you ask me.

When Naruto "met" his father in the Pain arc and also later when he "met" his mother it was revealed that Minato only sealed half of the Kyuubi in Naruto and the other half in himself. And now in the latest chapter that "shadow" of the Kyuubi has reawakened in Minato it would seem.

I say it's sketchy because back then Minato used the reaper death seal to seal those powers within himself, so the effect should have been undone when Orochimaru released all the souls and powers sealed within the shinigami.... or did he only release exactly those souls and powers he thought would be useful?