The Root of the Behavior
Charles Darwin wrote about survival of the fittest, and this theory certainly finds evidence in canine behavior. Dogs are descended from the wolf family and much of their innate makeup can be traced back to this heritage. Though today, we live with dogs that are far removed from the original wolf and his place in society, many of those instincts designed to ensure canine survival remain present in our dogs. Instinct cannot be denied. A dog selectively bred over years to assist his owner on a hunt will still exhibit these traits even if his purpose in his new home has changed. A leopard cannot change its spots, and a dog cannot deny thousands of years of genetics who have made him who and what he is. So though some of our furry friends' behaviors can give us cause for concern in terms of living in domesticity, it does not mean that those actions are wrong or even abnormal from the perspective of the canine.
Often this behavior rears its ugly head when two dogs who have lived in harmony for many years suddenly have a spat. Most often it is a younger dog attacking an older one or one who is physically compromised in some way. Why does this happen? The answers aren't clearcut, for sure. Many assume that since wolves traveled and hunted in packs that a sick, injured, or old family member merely slowed down and drew unwanted attention to the group as a whole. Since canine survival was dependent on remaining safe from predators and ensuring a plentiful food supply, dogs that could not contribute to protecting and hunting for the pack were eliminated so as not to drain or require additional resources. Though this is certainly possible, it is far more likely that there is some sort of physiological change in the vulnerable dog that sets off this ancient reaction in its housemate.
Encouraging the Behavior
While we do not want to encourage this behavior in our dogs in any way, there is little we can do to change instinct. What we can do is understand the "whys" behind it and take every precaution to protect our old, sick, or injured dog. The best thing we can do to prevent these things from happening is to stay aware of the potential problems and to supervise all dog interactions. If you cannot be there to actively watch your vulnerable dog when he is with other dogs, it is best to crate him in a comfortable location where he can rest without fear of conflict. To allow him sufficient time with you, you can also rotate crate time so that the younger dog also has some quiet time in his crate to allow your older dog the comfort laying on the sofa with you for awhile while you watch TV.