Whedon’s involvement with "Justice League" seemed to be in pursuit of crushing Snyder's vision in favor of his. There were suppoedly endless rewrites of Snyder's original screenplay, reshoots, etc. All this is an attempt to course-correct the DCEU. There have also been rumors that Snyder was actually fired and didn't just step down due to personal issues. I must, however, command Snyder for being rather quiet and unpronounced through this whole ordeal. The WB has shown interest in a possible Snyder Director's Cut of the film, especially after a petition with hundreds of thousands of signatures was unleashed to the public last year.
Fiona Zheng, who started the the Snyder Cut website, shared a photo of a tweet reporting on Whedon quitting "Batgirl" after creative differences. Her caption reads, “For today, the only bad news is these media/press still think Whedon out was bad to DCEU!” A post which Mr. Snyder actually liked.
According to Screenrant: “Whedon did not (in the words of producer Charles Roven) “help Zack finish his vision.” He massively overhauled the team-up film, eschewing the tone and plot points Snyder had set up for his trilogy in favor of a more light-hearted approach that looked to ape The Avengers more than anything. Considering how things turned out, it’s understandable (and, quite frankly, human) for Snyder to take a little enjoyment out of Whedon’s Batgirlfalling through.”