Soo... i just had an ephiphany:
Madara probably killed himself.
We know from chapters 560 and 561 that Madara gained a portion of Harashima's power, thus allowing him to advance his Eternal Mangeyko Sharingan to the Rinnegan.
We also know, both from the manga and the anime, that he was severely weakened at that point.
Here is where we get into speculation.
Again, from chapter 560 we know that Madara's fusion with the Senju DNA was flawed. The implication is that if the fusion was perfect, there'd be no need for Kabuto to augment and perfect Madara's body.
Now, what happens when someone pushed to their limit tries to use a Sharingan and senju-chakra power?
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Notice the giant tree erupting from Danzo's body.
So the real question is, do we have anything resembling that elsewhere in the Narutoverse?
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we have Tobi, posing as Madara, stating that this is the mass of cells that he stole from Harashima.
Odd that it's formed a giant lotus blosom that supports Gedo Mazo.
[Baseless speculation]
Almost like someone was trying to make the Gedo Mazo with a new found power without realizing the consequences.
[/speculation]
Madara used his Rinnegan to do something and the new Harashima DNA went crazy. The result produced the White Zetsu.