why obito is possesed by izuna
Here are the reasons that izuna had somehow possessed obito's body
http://www.narutobase.net/manga/Naruto/399/4
Izuna was hurt talking about himself cause the memories were to much and he was clenching himself
http://www.narutobase.net/manga/Naruto/597/12
Notice he says only after the fight if it really was obito he would have said he had it all along being his eye which means that izuna possessed him on that day
-Then their is the stuff he has done and knows
-Izuna would hate the leaf for it being senju ruled and what happened to his brother
-Izuna would know all of this stuff about the ms and ems as he was the second person to awaken it in history so he must have awakened obito's in another way after he possessed him
-Tobi and madara are working with each other so it would make sense that tobi is izuna as that would be the only person he trusted with such a thing
-It is possible that izuna was revived in some form as madara had the rinnegan and when he was revived expected nagato to revive him which means you could be revived long after you died.
Of course their are holes like why he was in obito's body or if he was revived prior what body did he have and it is possible that madara lived until the third shinobi world war and used the rinnegan to put izuna's soul in obito's body.