Why do cows eat bark from trees?

Normally they do not. Usually the main culprits for stripping the bark off of trees are moose and deer in the winter when they have nothing else to eat. But if they were to it likely was from a phosphorus deficiency...or that they were starving and had no other food source. Cows with a P deficiency will also chew on wood fences or eat soil just to get the phosphorus they crave.