What would cause a dog to die 24 hours after an attack?

My parents dog was attacked in their yard by 2 other dogs (he's an outside dog). By the time they got out there, the dog was lifeless. After a few minutes it got up with no visible blood anywhere. They noticed he was wobbling when he was walking though. He made it through the night and asked to go outside in the morning. During the afternoon, he walked into the bush and died. What would cause him to die so long afterwards? Is it possible that these dogs came back to finish him off?