The cry does give Thor a moment of conflicting thoughts, it's hard for him to justify the warpath he's on but his pride is calling the shots here. Unfortunately, that brief moment of indecision gives Bucky the chance to grab onto Thor and pull him backward.

Thor lets out an unflattering grunt of surprise as he lurches, but he's quick to grab at the mechanical arm. "I care not for your missions!" If can take a hold of it, he'll grab onto it so tightly that the powerful metals threaten to bend under his grip. That's when he'll throw the first punch, right at Bucky's face. He does like the fellow, but he doesn't much care for his attitude.