If they were only protecting their territory, why would they have been attacking the fox only when it was going after the turkey? Why wouldn't they harass it as soon as they saw it until it was gone like they do the hawks?
Once I saw a giant crow, it would have had a six foot wingspan at the very least, attacking a quite large echidna. An even bigger eagle swooped down and drove off the crow and tried to take the echidna. We ended up saving the echidna. In this case it was one predator seeking to take another's prey. Crows enjoy meat too, there's a good chance they hoped to consume the chook instead.
Many predators pass through their territories all the time. No matter what they do there will always be predators in territories overlapping the crow's; the best they can do without dying pointlessly is to persuade the fox that hunting around crows is futile. I think it's got nothing to do with the prey species, it's about the territory. Why would any bird intervene to save or avenge a mutual enemy? But they do. Some of these enemies kill baby crows, so it's not like they co-habitat peaceably, it's just that there is a greater foe they can exile when working as a team.
Every season, many young predators born in or near a crow's territory must move outwards to find a new territory of their own. Crows would be wasting a lot of time chasing them all if they bothered. Animals often don't bother wasting energy when the threat is not immediate, or when the risk outweighs the benefit. Like when a lion's killed a zebra, the herd stops and rests, close by, knowing they're not a threat now. Attacking a dangerous predator when it's obviously just moving on through the territory could lead to it making a kill of one of its attackers. Once it's attacked something though it's crossed the line, that seems to enrage birds.
Often I do see crows etc harassing predators out of their territories, mainly hawks and snakes, etc. But often birds just sit and make the alarm call, not bothering expending such energy on a walk-through. If the predator attacks, then it is worth attacking, and often less able to defend itself as it struggles with its prey, which makes it a wise investment of time and energy. Crows are very intelligent and have strong family bonds, they don't like to risk their lives for nothing, same with kookaburras etc, in fact most birds that invest time in keeping away predators have strong family ties; those species that don't are usually those that stay safe in the trees yelling about it.
I don't think there's any altruistic motive towards chickens in particular, it's more of a personal territorial-protective reaction about the predator's decision to hunt in the crow's territory, in my opinion. If the fox kept going, the crows wouldn't have had to teach it a lesson. Most foxes don't seem to bother hunting in a crow's territory because of this reaction. Some birds will live a fairly peaceful life compared to others because all the predators know if they're spotted here they'll be loudly named and shamed until they leave, and other prey animals know those alarm calls and pay attention.
Mom reached up and bit him on the tail as he swooped down and tried to drown him. Yes, drown him by dragging him repeatedly under the water!
I don't think anyone disbelieves you, lots of people on these forums have trouble with their ducks dragging their chooks into wading pools and drowning them.