Why did all these cows jump off a cliff to their deaths?

Last month, a dozen young cows in Levron, Switzerland mysteriously leapt off a cliff and plummeted to their deaths 165 feet below. A thirteenth cow survived the jump by landing on the others. From Mysterious Universe:

The calves, 13 Hi(C)rens aged 6-8 months, were found at the bottom of a 50 meter (165 foot) drop and all were dead except for the one on top, which was taken to an animal hospital in nearby Cern where it was treated for a broken jaw. The rest were cremated at a meat waste facility, to the dismay of a local animal warden who wanted to inspect them to help determine why they leaped to their deaths...

Norbert Terrettaz, president of a local farming insurance company, suspects the cows were chased or spooked by a wolf, a lynx or a dog. However, that doesn't explain the fact that there were no scratches or bites on the corpses, no tracks or spores on top of the cliff and no explanation for why the yaks didn't run too - either off the cliff or, after hearing the splats, in the opposite direction.