When I have more than 8 to 10 cows in a pen, and either add water inside the pen in a corner, (like an infinite water source) or go away a few chunks, i noticed that after a while the cows will just cram in one corner and stay there. Even when I get hay and lure them out, they just go back to the corner. Only if i breed and then kill a few cows, they will get out of that corner. In the screenshot I have sheep in the pen too, to show that it only happens with cows.