Yeah I agree^.Shisui father isn't alive at the time of the Uchiha slaughter, So I am going to assume he was really close to his father and seeing him die unlocked it.
Or maybe his girlfriend, brother, or something died. Anyways point being, we don't know but since his father wasn't alive I am going to assume he got it from seeing his father die.
ALL HAIL UCHIHA!!! Thanks for reppin the Mighty Uchiha Clantrue, i'd like to know the facts behnd how shishui got his MS aswell.