I think this explains it right here. As another commenter said "more fear than anything else". Animals act very differently than humans when they are scared, they often get very aggressive. Anecdotally, when I was younger my loving smush of a dog got hit by a car and I ran over to her and she bit the shit out of me. She was scared for her life, and that's just how her brain was wired to react.

And just so I don't leave anybody feeling awful, she made it to the vet, needed a pin in her hip, and her tail was amputated, but she went on to live to the ripe old age of 15. My bites weren't too bad because she was a small dog. No stitches needed, but I have some tiny scars left if you look really close

But if you want to feel angry about the situation, it was a cop car that she was hit by which was flying down a residential street, and the cop yelled at me and my mother and threatened to give us a ticket for having a dog off the leash. And thus my hatred for police began at the age of 10.