How do i get my dog to stop attacking my other dog?

I have a 5 year old lab that has always been a extremely passive dog. In the last 2 months she has started to attack (very violently)my other dog(also very passive)by grabbing her neck and shaking. of course my other dog fights back and a war starts.She will not listen to me at all and i have to step in and physically get her off. I am very scared that she will kill my other dog if I'm not there to break it up. They have been together for 3 years without the slightest problem and are still fine accept for once in a blue moon with no warning.What would cause this to start all of a sudden?