Tfue is one of the best players you can find in Fortnite, but since the release of Chapter 2 it seems he hasn't been fully satisfied with the changes and new features Epic have introduced.
Now, with Call of Duty: Warzone in full swing and Season 4 about to begin, Tfue has moved on to the Activision's battle royale because he feels that Fortnite is an unfair and unbalanced game.
He's not going to quit Fortnite permanently, but he's probably going put off his return for a while.
Of course, Tfue has more than just these reasons to put Fortnite aside. He's been playing this game for many years, and Epic are facing more and more criticism from the community about the state of their title at the moment.
Updates don't come regularly anymore, we don't have patch notes available, and events are postponed several times within a few days. This is not even to mention the competitive side of the game and problems related to cheating or fairness between players.
Despite wanting to try something fresh with Warzone, chances are he'll return to Fortnite sooner or later.