Dog accident at pets hotel..?

Do they let the dogs out together? Or if they have adjoining chainlink type kennels, yes it's possible your Boxer was in a scuffle with another dog. It is also possible she broke the tooth & cut her lips biting on the kennel or crate. Ask Petsmart what happened. Their liability would depend on what kind of release you signed when you left your dog for boarding. If the tooth seems painful or sensitive or the cuts start to look infected, definitely get her into your Vet for exam & care.