Madara state's that Obito's body somehow "slipped" away from rock that should have killed him. He also states that he found Obito's body NEXT to the rock not beneath it. Kaumi is the only reasonable explanation of Obito's survival. His death caused him to believe that he has lost Kakashi and Rin because he was unable to protect them which triggered the activation of his MS right in the brink of death. Besides Madara was too weak to recover Obito from the rock himself.
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