Younger dogs attacking older dogs... Why?

I have four dogs a senior (15 years old) neutered male American Pit Bull Terrier, a senior (14 years old) spayed female Golden Retriever, a young adult (2 years old) spayed female Akita, and an adult (four years old) neutered male American Pit Bull Terrier. I have had all of them since they were 10 to 12 weeks old and i have never had a problem until recently. My two younger dogs have recently started to attack my elderly dogs to the point of blood being drawn, the attacks are completely random and happen a few times a day, for the last five days i have been keeping my elderly dogs separated from my younger dogs since the last fight ended in my old Pit Bull needing staples to close the wounds. They are all fed separately and they have nothing at all to fight over. My two younger dogs will just quickly glance over at the elderly dogs (Even if they are sleeping or not even looking at the younger dogs!) then leap on them and attack. Do i just need to keep them separate? Does any one have any idea what could have caused this very sudden change?