- personally i think because he was dying and eager to escape he somehow managed to use his space time technique without even knowing he did it or he could do it...
- also i read i few pages back that madara awakened his rinnegan after battle with hashirama what possibly is true but i was just thinking maybe nagato was a little 'experiment' for madara and he gave his 2 sharingan eyes to nagato which then became rinnegan due to the fact that nagato is an uzumaki and therefor also a relative to the SO6P... and because of that the power of the eyes (sharingan/uchiha) and the power of the body (senju/uzumaki) came back together and his 2 sharingan became rinnegan.
(again just a theory i'm not saying it's true but the credibility is the same as madara awakening the rinnegan after fighting hashirama, or in this case the body and eyes of the SO6P come together what creates the rinnegan)
so far my post, next few lines is just some WILD guessing so please don't make a fuss about it only what i said above this line is what i actually think is possible
- BUT

if my theory is correct it would mean that with naruto being a uzumaki he would also be able to get the rinnegan if received a sharingan :O
aaaand imagine now a scenario where sasuke is dying or working together with naruto it could be that naruto gets his eyes wow

anyway just brainstorming here haha
