In this case it seems like director Josh Trank may have been in over his head. Word is that the studio stepped in to salvage the movie because they lost confidence in Trank and his vision. Trank was characterized as
"eratic", "isolated" and "uncommunicative". Multiple sources on the project got the sense that Trank was not offering them clear direction, despite their impressions that he is a very talented filmmaker. Josh Trank, or perhaps his dogs, was also accused of $100,000 in damages to the rental property where he was living while FF was in it's principal photography stages.
That THR article also states that producers Simon Kinberg and Hutch Parker had to step in to help pull the film together. And it's very likely that given Kinberg's close association with the new Star Wars regime and his experience with Josh Trank on this
Fantastic Four project, that is why Josh Trank was pulled off of the second Star Wars anthology film.
In the meantime, Josh Trank is being petulant. He posted to
Twitter that he had a fantastic vision for the FF film, but unfortunately people won't see that. He quickly deleted that tweet, but basically accused Fox of meddling in the process. But looking at this from an outside perspective it seems as though Josh Trank wasn't prepared to step up to direct as big of a project as this FF reboot with a clear, defined vision.