Regardless of what the law says, the victim always has to prove why the dog attacked him. Even in a strict liability state, you have to prove that something other than the victim's own conduct caused the dog to attack, because judges, juries and insurance adjusters tend to believe the cultural myth that all dogs are good dogs and that no good dog would maul a person without a good reason. Because dogs are regarded as "man's best friend," everyone (the witnesses, the adjuster, the judge and the jury) will put your client on the list of suspected causes. This is totally unfair to the victim because legal provocation only occurs in 6.5% of dog attacks:
Under principles of Common Law there is the assumption that dogs are harmless unless they have previously demonstrated a vicious propensity. This often leads to the related assumption that victims of dog attack have provoked or otherwise precipitated the attack. However, those studies which have attempted to document the context in which an attack has occurred generally show that bite victims are rarely engaging in activity that could legally be considered provocation (i.e., causing physical injury to the animal). In the non-fatal bites surveyed by Beck et al. (1975), the victims had no interaction with the dog, or were walking or sitting in 75% of the cases. In 9.6% the victim was playing with the dog and in only 6.5% could the victim's behavior be classified as provocative. (Lockwood, The Ethology and Epidemiology of Canine Aggression, in James Serpell (ed.) The Domestic Dog: Its Evolution, Behavior & Interactions with People, (Cambridge, U. K.: Cambridge Univ. Press), pp. 132-138.)
You cannot permit uninformed myths about the presumed friendliness of dogs to denigrate the value of your case. Therefore you always are tasked with proving the real reason why the dog bit the victim.
There is another reason to get into the cause of the attack. You have heard the expression, "It's a dog's life." While some people treat their dogs literally like kids, others habitually neglect them, even to the point of mistreatment. For example, many owners do not bring their sick dogs to the veterinarian. A dog with a painful hip may snap at anyone who touches the hip. Showing that the dog bit the victim because the owner neglected or mistreated it casts moral blame on the owner. This enhances both the liability and damages sides of your case regardless of whether your case is governed by a strict liability dog bite statute.
One of the most important goals of the dog owner's deposition is to learn as much as possible about the dog. As you probe the dog owner, you often will learn that he neglected or mistreated his pet, that it was uncared for or sick, that it was often chained, or that he employed it for protection or even intimidation. With such evidence you eliminate the risk that a jury will reduce the victim's compensation because of speculation about provocation, and you enhance both the liability and damages sides of your case.