A dog owner can be held liable for damage in any case where he agreed to accept the liability. Acceptance can be by contract, written, oral or implied. Here is an exchange of email between a dog owner and Attorney Kenneth M. Phillips regarding something that happened at doggie daycare:
[Dog owner:] I left my dog at doggie day care. I have left my dog over 20-30 times at the same place. They called me today saying that my dog was bit another dog. My dog was defending itself and it was bitten. They are saying I am liable for the other dogs expenses because I signed the doggie day care liability form. Apparently they said my dog is skittish and sometimes was aggressive but they had never once informed me of this behavior before.
[Attorney Phillips:] That's terrible. I bet you are asking me whether you are legally responsible. The answer is yes, if that's what the contract says. We use contracts so that we can accept or reject the general legal principles that would govern events otherwise. If you agreed to be liable, then you're liable. But be sure to confirm it by ... reading that contract that you probably signed without reading!