Disney released the following statement:
Lucasfilm and Colin Trevorrow have mutually chosen to part ways on Star Wars: Episode IX. Colin has been a wonderful collaborator throughout the development process but we have all come to the conclusion that our visions for the project differ. We wish Colin the best and will be sharing more information about the film soon.
Star Wars: Episode IX doesn't have a new director as of yet, but it's said Star Wars: The Last Jedi director Rian Johnson is eyed for the gig. The movie is due to open May 24, 2019.
Regarding what happened with Colin Trevorrow, it's said to be because of "creative differences," but it seems as if the directors that are leaving the Star Wars projects all have one thing in common, which is that they are new.
LucasFilm has seen a number of directors leave including most recently Christopher Lord and Phil Miller off of Han Solo with Ron Howard replacing them, Josh Trank from a Star Wars spinoff, to an extent Gareth Edwards losing directorial control on Star Wars: Rogue One to Tony Gilroy, and now Colin Trevorrow leaving Star Wars: Episode IX.