Did someone kill after he defeated madara when he was at his weakest ,or maybe when he weaken madara the jutsu also weaken as a double edge sword ,or perhaps he got jump like his brother.
Kishi will probably reveal only how he fought Madara and not how he died. It actually doesn't really matter to the story, so he will let us imagine it as we wish. Sometimes it's best to leave things like that.
it maybe after effect of madara battle.If Hashirama know mitotic jutsu like tsunade to tank madara jutsu,it will damage his health afterward when deactivated