That's the question pondered by farming experts in the area, who fear the animals may have been chased by an unknown predator, reported Swiss media.
The 13 Hérens cows broke out of their pen, ran towards a cliff and either fell or threw themselves off the 50 metre drop last Wednesday night near the village of Levron, local paper La Nouvelliste reported.
Twelve of them died, the thirteenth survived -- probably because it landed on the others -- but was injured and taken to an animal hospital in Bern.
“The cows broke through several lines around the pasture before finding themselves on the ridge. They fell into the void, as though they were forced into it,” Norbert Terrettaz, president of an farming insurance company in the region, told the paper.
In total the cows were thought to be worth 20,000 francs.
They belonged to several owners but were all grazing on land belonging to a local couple, who said they thought some kind of predator must have chased the beasts off the cliff.
Nevertheless, their behaviour was unusual.
“Cows don't follow each other like sheep, when they are scared they have a tendency to disperse,” said Terrettaz.
Perhaps a dog off the leash provoked their flight, or even a wolf, he said.
Speaking to Le Matin, another expert agreed it was puzzling.
“They must have been blocked somehow. One or two cows falling off, that's possible. Thirteen, that's a new and incomprehensible phenomenon,” said Benoît Berguerand of a breeders' association. “It is unlikely that they threw themselves off without pressure."
To add to the mystery, before reaching the cliff the fleeing cows had to pass through a pasture occupied by four yaks, none of whom had injuries consistent with a predator. If a wolf or lynx was pursuing the cows, it's unlikely it would have been uninterested in the yaks, pointed out La Nouvelliste.
Since there is no evidence to point to the cause, the commune is now appealing to witnesses to help explain what might have happened.
The Hérens cow. Photo: Valais Tourism
The Hérens (or Eringer in German) cow is a traditional breed in the Swiss canton of Valais whose ancestors existed in the Rhône valley region as far back as 3,000BC.
They are bred throughout the Valais, particularly Nendaz, Nax/Mont Noble, Veysonnaz, the Val d'Anniviers and the Val d'Herens, plus Leukerbad on the other side of the Rhône valley.
Naturally combative, the cows often fight to establish the leader of the herd, behaviour that has led to organized cow-fighting competitions in the area.