I suspect it very much depends on the initial conditions. A Zebra that decides to try to scare off the Lion, but isn't backed up by the rest of the herd, or isn't backed up by enough of them quickly enough, is dead meat. The problem is their ability to communicate with each other and co-ordinate is too limited.

" A Zebra that decides to try to scare off the Lion, but isn't backed up by the rest of the herd, or isn't backed up by enough of them quickly enough, is dead meat. "

Not necessarily true. An injury by a kick is a huge problem for the predator so they often shy away if the animal fights back. I have seen plenty of videos where single zebras or deer chased a predator away. Obviously this changes if there are several predators.

The trouble is Lions generally hunt as a pack.

Some kind of result of co-evolving? Predators thinning out weaker members of the herd keeps the overall health of the herd up.

Just some quick speculation.

Reminds me of the Battle at Kruger:

https://youtu.be/LU8DDYz68kM

maybe a hormonal variable at play, anybody can fight when enraged, even if on average they might flee

For social animals like chicken, sheep, even humans the action of the group is often determined by a few - the leaders or the first to act.

If the first chicken panicked and ran, the rest would. I'm guessing the fox ran into the aggressive ones first.

To put humans into context here. Once, a couple of bombs went off in an Amory. People panicked and ran. Everyone ran in the direction of those who ran first.

Long story short, a few died from the blast. But, hundreds died from falling off a cliff and drowning in a canal.

For animals where leadership is whoever happens to be in front, rather than a specific animal in their social ranking, you can get an hilarious situation where the herd has looped back on itself, so no one (or everyone, depending on how you look at it) is leading [1].

[1] https://www.youtube.com/watch?v=VJuS2ThE2q4

All those people didn't know how to swim? Or was the fall too high?

Drowning humans latch on to whatever's near out of instinct. Some portion of 100+ doing that will get a lot of good swimmers killed.