Nerdist ran a piece
that mentioned some major discord behind the scenes of the Trank helmed FF movie. And there's been a healthy amount of speculation that the FF reboot could be a bomb. Considering those factors, you have to think that Kathleen Kennedy and maybe even Bob Iger lost some faith in Trank's ability to deliver a film worthy of a place in the Star Wars canon.
And then when you add in this new rumor that Joss Whedon has expressed significant interest in directing a Star Wars film? You get the sense that Joss taking on this project became much more likely.