My guess is that Wizards of the Coast sold the novel rights to Warner Brothers as part of the merchandising rights to the upcoming film.
No big studio wants to make a movie when they can make a movie AND action figures AND video games AND apparel AND tie-in novels AND comic books AND lunch-boxes AND birthday-party plates AND etc. etc. etc.
So it's possible that Wizards has the right to continue producing their current D&D action figures/video games/apparel/novels, etc., but not the right to produce new ones. Warner wants to control that, to maximize their merchandising. And because Warner has a publishing arm and a well-oiled merchandising machine, they're going to use that to decide what novels to write and who should author them.
For a concrete example of how licensing rights can flip around like this, when Cryptic Studios acquired the rights to Champions, they got all the rights. Then they licensed the right to produce the RPG back to Hero Games. It's because video games are much bigger business than RPGs, and nobody's going to invest a ton of money into a video game without having solid control of the rights.
It honestly wouldn't surprise me too much if Warner Brothers now owns all the rights to D&D and licenses RPG production back to Wizards; although Hasbro has a lot more clout than Hero Games so it's possible that Warner only has film and merchandising rights.