Thor: Ragnarok - 10 Huge Spoilers From The Comic Con Trailer

6. ...Bruce Banner Has Been Out Of It For A While

Even more interesting than Hulk speaking, the trailer reveals that Bruce Banner has no memory of anything that has happened recently (including immediate events like the arena battle with Thor). He apparently no longer retains any memories of when he Hulks out, which is an interesting new development that seems to suggest that his Hulk alter-ego is gaining in strength in their internal conflict.

Perhaps he is kept under control by The Grandmaster, who keeps him perpetually triggered to make sure he doesn't devolve into the more pragmatic, calmer Banner? Maybe Thor's first act after the pair fight (or during it, even) is to break Grandmaster's pattern of control and freeing him?

That would explain how he becomes human again...