This is from another dog forum about a somewhat similar situation and the bottom is a link to another article
With dogs kept loose in a pack situation...this isn't uncommon. Even highly trained working dogs (sled dogs) start turning on the oldest after a time. That's how retirement ages are decided for sled runners...not so much by their physical limitations but when the rest of the pack starts considering them obsolete. Because a pack does not survive/work well when dogs age out...it's not the other dogs being "mean" but their own survival instincts kicking in. Domestic dogs kept in pack situations revert back to pack mentality...their instincts become stronger than their training.
Dogs kept outside and then away from household rules and humans all night are considered a pack and not really a bunch of pets. At least not to the dogs. Unless they're being worked by humans a good portion of every day following specific protocol and commands...the pack instinct takes over.
Your older dog may have slowed down just enough for the other dogs to notice...but not enough for you to really notice yet. There may be a health issue not seen yet...there may not. Even a dog's scent changes as they age, other dogs will notice. Being in a pack means the younger dogs will start lobbying for pack positions and will use the low man on the totem pole to practice their bids for alpha on. Low man always gets the most unprovoked knocks...that's nature at work. Any changes in the dynamics or maturity of the pack will affect the low man even if the low man hasn't done anything to "deserve" it.
This is when the humans have to take over and start managing the pack. Either change their entire daily routines and get them back on obediance schedules...ie re-establish yourself as leader and then lay the ground rules....or you have to separate the old dog and start keeping it protected. Because one attack means another will definitely happen. There were signs leading up to it...which is usually the time to remove the older dog from the pack for it's own safety. The oldest dog was very lucky that it wasn;t harmed so far. Old dog needs to become a house pet...and when outside it needs to be supervised/kept from roaming free with the rest. Because packs can act sneaky...everything will seem fine and they will all turn at once as soon as they can. And the old dog might not survive the next one. Getting rid of the instigator won't work because a new one will take it's place unless their daily routines change.
Dog Behavior Problems - Aggression - Sibling Rivalry - Diagnosis | VCA Animal Hospitals