The article mentions that the walker was with his Dogs, so its a distinct possibility that the cows became startled or threatened by the presence of a barking dog.
The majority of deaths, do seem to occur when walkers are walking with their dog, either on or off their lead.
Its within a Dogs instinct to bark or get excited when in the presence of other animals, and a large herd of cows can often feel threatened when a barking dog is nearby.