Sudden Onset of Aggressive Behavior in Lab

Question:
 I am 51 years old and have had dogs all my life. I train them with love and affection. I pride myself on having the mildest-mannered, gentlest, loving pets. I currently have three dogs. Yesterday, one of them almost killed another. One of them (Red) is a 15-year old, neutered, 80-pound Black Lab male who has never exhibited any type of aggressive or defensice behavior. He is the opposite of an Alpha Male. I have always said that he is the gentlest animal God ever placed upon this earth. He is a descendent of two Field Trial Champion parents. He was somewhat of a Velcro dog in his earlier years, and has always been the penultimate lover toward every creature. One (Harley) is a 6-year old, neutered, 93-pound, very fit Black Lab male. I have occasionally heard him give a low growl from inside the house when he suspects there is strange wildlife around our fenced-in back yard. He will also go out the doggie door at night into the fenced back yard and bark for a short period at wildlife that may be around the house. If off of a leash (which is very uncommon) he will run after deer, although I have no idea what he would do if he were ever to catch one. If he is alerted that someone is coming to the front door, he will occasionally bark once, in an effort to get to the person ASAP to lick and cuddle him to death (stranger or friend). I have never seen any other type of "aggressive" behavior from him, and he has been a constant gentle giant, and big brother to the other two. An exception is when throwing a ball to Red, Harley will try to intimidate Red to not go after the ball (whether on land or in the water). He will essentiall block Red from going after the ball or bumper. One (Georgie) is a 2-year old, spayed, 60-pound Pudelpointer female. Although a high-end bird dog, I do not hunt. She is a pet to us. She, like Red, has never displayed any aggressive or defensive behavior (she will sometimes carry one of her toys in her mouth while giving a playfull, higher-pitched howl saying, "look at me!" Compared to the other two boys, she is quite spoiled by my wife, and actually sleeps on our bed with us (I haven't been effective in training her, as consistency is not one of my wife's fortes. Being a bit more high-strung, she will bark for a short period whenever she thinks she hears something, and then she runs to the happy, tail-wagging security of our bed. She acts as Harley's first alarm, as if to say, "OK, Harley, I heard something. Now you go and check it out," Although moderately food-oriented, noner has demonstrated aggressive behavior to another on the basis of food. Yesterday, my wife heard a dogfight occurring in our fenced back yard. Red was howling in pain, and Harley had him pinned to the ground with his mouth around Red's neck. There was a little blood from some punctures on Red's neck. I was not home, but she said she did not see anything that might have caused this unbelievable altercation. After calling me at work, we decided that there was nothing else for us to do, and that this unbelieveble occurance was over. She left the house for a few hours afterwards. When she arrived home, the house was full of blood. Harley was inside bleeding from a torn ear and a laceration on the cheek below an eye. She found Red outside, lying in blood, almost unable to move, almost exsanguinated from dozens of deep puncture wounds to the neck. After a few hours at the Vet, we brought them all home. Except for Red's extreme pain from the deep neck puncture wounds, neither Harley or Red seemed the least wary of each other, and they even "spooned" a bit this morning (big, bloody, black balls of fur that they are). My wife and I are now terrified to leave them alone. We have never thought twice about leaving them alone for 12-13 hours before (they have their doggie door to go outside). The only possible thing we can think of is there was a bag of rawhide chew strips (my wife purchased them, as I prefer not to give them rawhide chew strips) that had "spilled" on the floor while we were away. A number of them were carried outside by one or more of the dogs. It is possible that there were some unseen chew strips that were the source of the fight, but this seems so out of character that we are in total disbelief. Can you help us and/or our doggies? Please.
Answer:
 Thank you for your words. I guess no matter how docile pets are, unsupervised treats within the pack can cause the Alpha to assert itself. I also suppose that, since I was not around to be the Alpha, the next-in-line did what an Alpha does. It is still terrifying to me that my gentle giant has that capacity lurking somewhere within hime. After 6 years, I'll never be able to look at him the same. Maybe I was looking at him the wrong way all this while, since, regardless of my anthropomorphizations, Harley is a canine.