If I had to put the story together, I'd say he fell off the horse into the forbidden area. Except the sign is on the other side of the wall, indicating that's the no-trespassing zone. Or he leaned over the fence and the horse, which is actually a donkey, kicked him in the head - but if that's so, and the man in white is mad, what's he doing on the other side of the wall?

If the man in the black coat and spats was riding the horse, what was doing with a walking stick?