From what I've heard (no idea if it was correct or not), Snyder was already on the outs with Warner Bros. over BvS, which underperformed compared to the recent Superman and Batman solo movies (especially given what one might have expected from first appearance of DC Comics' Trinity in one film), and was then shown up by the overperformance and near-unanimous critical acclaim of Patty Jenkins' much more colorful and optimistic Wonder Woman. Then it was looking like Justice League was shaping up to be more of the same grimdark violence-palooza, and Whedon was brought in to Avengers-ize the project with more humor, quippy dialog, and brightly colored action in the hopes doing so would add another billion or so at the box office. The tragic death of Snyder's daughter gave the studio an excuse for his exit that would save face publicly and avoid animosity in the future.

Turns out Justice League underperformed even worse, was also critically panned, and in the meantime all kinds of unpleasant revelations about Joss' personal and professional conduct came to light just as the MeToo movement was making such things problematic for studios. Meanwhile Snyder's fans who thought Batman v. Superman was a cinematic masterpiece have been calling for the Snyder Cut for years, and to his credit Snyder seems to be well-regarded by all the principal actors who've dealt with him.