This is a bird called the Sereima, and what you are seeing isn't random behavior: it's how they hunt. They chase rodents and lizards down, grab them with their beaks, then slam their prey down onto rocks repeatedly until they stop moving.
Metal as hell.
We used to rent our farmland to the neighbors for their cows and horses. We would periodically have to go out there and do work on fences and the barn and stuff. I was always told that horses are not to be trusted, but the cows are probably fine.
Take that information and do with it as you will.