HR is saying script issues on Episode IX forced the split.
Sources tell The Hollywood Reporter that script issues have continued to be the continuing sore spot with Episode IX’s development with Trevorrow having had repeated stabs at multiple drafts. In August, Jack Thorne, the British scribe who wrote the upcoming Julia Roberts-Jacob Tremblay movie Wonder, was tapped to work on the script.
Sources say that the working relationship between Trevorrow and Lucasfilm head Kathleen Kennedy became unmanageable. Kennedy, who had already been through one director firing/replacement on the Han Solo spin-off, was not eager for a sequel and tried to avoid this decision.
I assume part of the difficulty with Trevorrow is that he started with a treatment that involved whatever plan Lucasfilm had for Leia’s role in the final confrontation with Kylo Ren. When Fisher died, the rumor is that a lot of that writing had to be thrown out and Trevorrow just couldn’t pivot fast enough. Throw in his Book of Henry performance, and Kennedy had good reason to get rid of him.