Love the photo! I have some similar ones.
I come across a lot of cows when out making photos or recreating and in general they are pretty docile. I love it when the young cows/steers are curious and come to see what I am doing. Only the bulls have ever given me any trouble and they are not typically the ones grazing on public land here in Colorado.
I suspect some kind of illness in the attack cows if it wasn't a bull protecting the herd.
Or maybe there are some ornery breed we don't get here.
I'm not a cowboy but I live among them.