Could it be as simple as familiarity with one another? Neither are a threat to the other and there is some kind of symbiosis there. The cattle indirectly provide food and cover for the hogs and the hogs warn the cattle of threats> Just thinkinTLM, I wondered about that. I can't tell if the cattle are actually protecting the hogs, or if the cattle are doing their normal routine and the hogs have just placed themselves accordingly in the group. This herd has calves and so they are going to be somewhat vigilant about the calves. I wonder if the hogs are seen as being like calves to the cows? I don't know, but they do seem to get along very well together.
On this particular property, I look for the cattle first and then look to see if the hogs are with them. Either the hogs are amongst the cattle or very close by when I have seen them.