Since Zootopia's Story Limit is broken, then based on what Chris says above, it may be possible that the second Optionlock introduced through Bellwether's scheming may have contributed to the unclarity in the Story Drivers. I wonder if, when he said that the Initial Story Driver was when Judy entered Zootopia, Chris was trying to take Bellwether into account or not, since the second Optionlock is outside of the scope of the story Zootopia initially sets out to deliver, with the Optionlock of the missing predators. I think that if the second Optionlock was kept within the other limit, like @jhull suggested, the Story Drivers would be clearer. What do you think?
Edit: Jim also says that the Initial Story Driver is the disappearance of the predators, which seems to be very close to what @mlucas said.