Musar על בבא קמא 120:8

There are yet three more signs of a rabid dog. His ears hang down, etc. That is, the rabid dog, aside from his bite being extremely dangerous, as mentioned by Chazal, has a great desire to bite anyone who comes before him. And so that people not hide from him because of the great danger owing to the evil spirit reposing upon him, as written there, he hides and conceals himself in every possible way, and suddenly he pounces upon the person and bites him. That is why he walks (and does not run) on the sides of the public thoroughfares and not on the thoroughfares themselves facing the people [(for that is where people walk, viz. Bava Kamma 60b: "A man should not walk along the sides of the roads, etc.")], so that people should think that he is going slowly on his way. And because of this, his ears also hang down. For it is well known that the Holy One Blessed be He has given a dog an acute sense of hearing to help him protect his master and himself from danger. And so, this dog, who wants to attack everyone, walks along the sides of the road with his ears dragging, so that he appears to everyone as going his own way and not wanting to hear anything on his sides. And that is why his tail also hangs down on his hindquarters, unlifted. All this to show everyone that he is quiet and secure and has no desire to rouse his senses to attack the people at his sides. And some say that he also barks without being heard. All because of the same reason, so that everyone think that he is the very best of dogs, and will not guard themselves from him at all. And all of these things he does through the agency of the evil spirit that rests upon him.