Let me put this idea out there. Bear with me here. Everyone talks about how the four element seal is really fancy and is very high quality. There's most likely a reason that not every country used it to seal their bijuu. Maybe it's a big secret but I don't think this is the case.
Rather I think that for the four element seal to work it takes sacrifices. In this case with the Fourth sealed the Kyuubi he needed a sacrifice. We know he sacrificed himself.
BUT here's where this Kushina theory could actually come into place. I'm not saying at all that she was the previous jinchuuriki.
I DO however think that she was a necessary sacrifice for the seal to work. The fourth must've used her body as a sacrifice while Naruto was STILL INSIDE her and that's why it's so high quality. She died and was sealed inside Naruto's mind with the fox.
What do you guys think? 100 dollars says that's exactly why she showed up.