One report stated that Josh Trank left due to creative differences, specifically with screenwriter Simon Kinberg, as the two of them apparently didn’t get along while making Fantastic Four.
Whatever the reason for Trank’s departure, the plans for the Boba Fett movie fell through, and the Han Solo movie was moved to that 2018 date instead.
Another report stated that Trank was dismissed after Lucasfilm learned of the director’s alleged unusual behavior during the Fantastic Four shoot and decided they didn’t want him in charge of this project.
However, behind the scenes, it seemed that this wasn’t a amicable split.