According to a very recent interview with Colin Trevorrow (May 8th) on AnimalTalking: He wasn’t fired. It was the bad reception of Book of Henry that prompted him to “hand over the ball” and leave
https://youtu.be/-Xl9F8bnJVM (50:00 - 52:00)
I find that highly unlikely. He had the perfect opportunity to redeem himself with one of the biggest movies of the year. More likely Disney said “hey we might fire you wink wink if you want to maintain your reputation maybe resign around now wink wink”
^ And as interesting as DOTF is, I don’t blame Disney for this, because I too would be nervous when the big finale is written and directed by a filmmaker who just had a major flop.