When we say that the festival was hosted on a farm, we literally mean a real, working farm. Real, working farms have animals. The animals were moved to a safe distance, but they were probably still a little bemused that there were thousands of people trampling their favorite spots of ground. And that's when they weren't bothering them for milk.
With nothing to eat or drink, people looked to the area around them for solutions, and unfortunately the cows came within range. People started visiting them for milk, and the equation of 400,000 people trying to get milk from a few regular cows doesn't work. Only the first few will have managed to get anything from them, but it didn't stop others trying. We should be glad that nobody decided to cook and eat them.