This isn't the first time they've done this. At times when they want players to take their time and enjoy something but feel there is something in the patch that would pressure them to rush, they'll divide it up. A good example is the normal 8 man raids becoming available before the savage modes.

We know they are tinkering with 24 man savage modes so it could be a similar release schedule. Alternatively, it could be the ultimate fight being released after the main part of the patch goes live so people can do MSQ at a relaxed pace before the race to world first starts. It could be another type of content altogether.