Since early Legion, Thrall has been out of the picture, disappearing when Doomhammer chooses a new hero. The Horde has had some hard times since before then though, thanks to him vacating his position as Warchief.
Saurfang has also been missing since a mission involving him in Swamp of Sorrows, where Horde players got to choose whether to betray or save him. Saurfang took the burning of Tel’Drassil about as well as most Horde players did, and he has been looking for answers ever since. Now he has found his old friend Thrall, farming far away from the wars and politics.
Many have believed that Thrall would come back, a notion that was given further fuel when a new model for Thrall was found in Rise of Aszhara PTR files. If he comes back, what does he come back as? This cinematic at least points to his return not being as Warchief.