Graves is a werewolf. Either he always has been one or was bitten in the line of duty. But the fact that he is a werewolf is top secret, only few people like Picquery know. Grindelwald certaily doesn't know and when preparing to impersonate Graves question "Do you happen to be werewolf?" just doesn't cross his mind. The nearest full moon whatever restrains he had on Graves doesn't work on a werewolf and he breaks loose. And maybe runs into Newt along the way - who is elated.