There are only a few bolts holding the back end onto the safety cell. It's designed to detach in a really big crash so that it doesn't impact on the safety cell if possible.
As far as I understand, yes, the car is designed to split under extreme circumstances. But two things to add to that: If the barrier hadn’t failed it might not have happened. And regardless, the ‘split’ is not supposed to cause a fire. The split happens right by the fuel tank and thus there were lots of safety systems that were meant to prevent a fire that didn’t do enough under these ‘extra’ extreme circumstances.