My 2 younger dogs have started to attack my 15 year old dog. What can I do?

Dear Mr. Frawley,

I hope that you can explain to me what is happening in my house with my dogs. Until three years ago I had four dogs - two Bichons and two Lhasas. The Bichons were two females who were six and seven years old respectively when the Lhasas were brought into the household at 10 weeks old. The Lhasas are male and female litter mates. All four were neutered/spayed. The 6 year old Bichon had a problem at first with the Lhasas but after about two months she warmed up to them and we lived happily for many years. Three and a half years ago that Bichon died at 10 years old. The other Bichon was 11 at the time. The three remaining dogs continued to live happily. The remaining Bichon is almost 15 now and deaf and almost blind. She gets around well enough though with the exception of some arthritis which stiffens her up until she gets going. She eats well and does not mess in the house. In the past year or so I have seen a slow but progressive change in her personality. She seems to be slowly regressing "mentally." The vet says she is healthy enough and as long as she is happy and comfortable, I plan on enjoying her companionship until the end. However, over the last 8 months or so, the Lhasas have become randomly aggressive towards her. Out of nowhere and for no reason one or both of them will attack her, knocking her down, making her scream and cry. I pull them away but they try very hard to continue the attack. I have immediately "gotten in their face" right afterwards by pulling the collar and pulling them close while I look in their face and admonish with "NO!" They immediately calm the aggression and everything normalizes until the next time. Unfortunately, there is always a "next time." When I'm away from the house, I no longer leave them all together anymore, leaving the Lhasas with each other, away from the Bichon. I don't get what's going on. Food has never been an issue, they are fed in three different corners of the kitchen, they eat their meals and the dishes come up. Why do the Lhasas attack a poor old almost defenseless dog that they have lived with all their lives? They are otherwise very sweet, gentle dogs. They have fought with each other on occasion but it is usually over a toy that is then removed from them both and the problem is thus solved. I know the solution is to keep them from the Bichon as I'm doing, but, I really don't understand why this has evolved. Can you explain?

Thank you,
Joyce