The dark side of the Hidden Mist
Before getting into the details, it’s important to understand what went on in the Hidden Mist village in the earlier days.
The Power dynamics was constantly shifting, a range of policies affected the other among other things.
Over time, many people doubted that the Mizukage was being controlled and Ao was the first one other than Kisame to realize this, thanks to his Byakugan.
But how was the fourth Mizukage die? Was he killed?
The possibility of death due to illness is remote as he was young and no illness has been portrayed.
Zabuza Momochi had once attempted a coup d’tat against Yagura and failed, hence he left the village.
He probably tried to kill the Mizukage again and succeeded. That’s one theory.