My first guess is that he's detecting something in the room that makes him uneasy.
Second guess is it could be a territorial behavior, defending the other chinchilla by holding his ground.
Third guess is something happened outside of the cage recently that caused pain or stress and he's afraid it could happen again.
Fourth guess is it could be a health issue, although unlikely. Just watch his weight carefully for a couple of weeks.
But as you know chinchillas are complicated animals. Maybe you'll figure it out and maybe you won't.