Would a 11 month old male Golden Retriever get more aggressive because you're in his pen?

My brother has a beautiful Golden. I went into his pen to pet him and he came at me very aggressively, biting at my arms and hands, growling. He never has done that. He likes to play rough but this was almost scary. I never thought he was going to get out of control, but he’s 93 pounds and strong. I backed him down and got control, but I’ve got some bruises and nicks to show for it. My niece said he acts the same with my brother and his son. Is this because it’s his territory?