A small study in Barcelona discovered that all twelve dogs who exhibited "unprovoked" or seemingly spontaneous aggression were in pain. Eight of the dogs suffered painful hip dysplasia.
While this does not provide sufficient evidence, in my opinion, to make any meaningful claims about dog bites, I think it is a step in the right directions in terms of studying animal aggression. It would be great if more universities created FREE research clinics for guardians of aggressive dogs.
It should be noted that the dogs exhibited aggressive behaviors before the onset of pain. However, the pain seemed to reduce any warning signals that may have been present before the dog became painful. This makes sense - a dog in pain is not going to warn you the same way a dog not in pain would. So a dog will suddenly bite for seemingly unknown reasons.