There was a group of people, who had concluded the best way to save Mother Earth and Mandkind, was to get rid of all that evil technology, and to let mainkind be closer to the animals. And for the animals to be closer to main kind.
How the organization did what it did was lost to history given the absolute chaos and social breakdown that was to follow, but the basic idea was this.
The scheme had taken a decade to put together, with scientists and engineers blackmailed or converted to the cause.
First had been a bomb that had been built, that would cause minimal physical destruction, but would send out an electro-magic-pulse of such power, that everything electronic on the planet would be render inoperable.
The second was a retro-virus that would spread through the human and animal population, and show no symptoms. Anything more complex than a microbe would be infected. Once all were infect however, an unknown single would be given... maybe pheromones, maybe some distortion of reality, it didn't matter, what matter was what came after.
Every animal on the planet, and every human on the planet, mutated at the same time as the bomb went off. Every animal was mutated into a humanoid shape, and (somehow) given human intelligence and ability to communicate the local language with no formal schooling what so ever. While humans were made half-animal hybrids of random creatures suited to the local environment, with all the instincts that had gone with it.
The result were predictable, but utterly lost on the group that had caused it.
Predators, did not stop being predators because they and their prey was now sapient, and human intellect, did not equal human morals, and humans were caught with instincts much more powerful than those found in humans and with no lifetime of experience on how to handle them. And with the animals made as intelligent and communicative as the former humans, there was no way to tell one from the other.
The group had been certain this would result in a vegetarian utopia free of the evils of soulless technology.
The result was a blood bath, with countless lives lost due to crashing planes, cashed cars, crashed trains, submarines, life support equipment, and that was just the initial loss of life.
Next came predators, with every food animal on the planet now sapient, and many now turned in pure predators (and animal predators turned sapient), the stocked food of store shelves went off the wall quickly and vanished.
Then came.... it wasn't truly right to call it 'cannibalism' anymore, since they were all obviously very different species now. They were ironically following their niche in the biological order of things, as the group naively believed in.
Social order broke down in days...
Of course... most modern humans (and animals) had no idea how to live middle ages style, lacking those skills that had been out of date for centuries... more died...
The same species grouped together, forming herd, packs, flocks, schools, and the like. Even more died as they fought for territory . . .
So much of humanity knowledge was lost, so much of their accomplishments were wiped away, so many hopes and dreams were annihilated.
The world was reduced to a tribal state.
Finally, prey species were reduced to less than slaves, with predators on top, and prey species were considered currencies (and so thing came full circle with the original meaning of 'capital' and heads of cattle.), they needed to eat to live after all.
The predator tribes guarded their territory fiercely, scavengers were considered higher than prey, but lower than predators. It wasn't quite right to call it racism, it was more like speciesism.
This was the group's vision of a 'perfect world' it had no turned out how they had hoped.