Aha, that explains it...

From The Soaraway Sun*:

SENSATIONAL declassified FBI files reveal how the White House was sent into a panic by suspected alien sacrifices of COWS.
More than 8,000 cattle were snatched by mystery aircraft, mutilated, then dropped from the skies above the US south west in the late 1970s, special agents told their directors.

Terrified farmers in New Mexico, Nebraska and Colorado feared they would be ripped apart by extraterrestrials in flying saucers. Details of the probe are spelled out in internal memos released by the FBI online.

The cattle killers took sick TROPHIES including tongues, lower lips, sex organs, eyes and ears. In some cases the doomed animal was drained entirely of its BLOOD...

And so on and so forth.

I think we might as well use this patently daft conspiracy theory to explain why cows are so violent against humans: either

a) they are getting their own back for us not protecting them against aliens, or

b) having done a few test runs, the aliens refined their techniques, kidnapped some cattle, mucked about with their DNA to turn them into cold-blooded killers and then returned them to their herds.

* Spotter's badge Sue