Ummmm probably cause back when Kabuto first met up with Obito, Kabuto(absorbed Oro) he showed Obito the Reanimation of Madara which Obito question "how did he get his body?" it could of meant two things though 1: Obito gave Madara a secret burial ground when he past and question how he found it or 2: Obito wanted to be the only ruler of the Moon eye plan so he killed Madara and stashed his body some where so he ask how did he found it

idk probably took it too far but it's what I think if he did killed him xD
I mean come on if he didn't than he would of been happy to see the reanimated Madara don't you think? instead of being shocked and piss scared. Plus when Naruto mention "other Madara" during Thier first battle in the War he mumbled or whispered "damn that Kabuto" yeah so I guess your right in a way maybe

damn should of made this a theory thread -_-
Can't explain right on this iPod x(