I love the idea of cows popping into unexpected places, this is a perfect example.  Imagine, you are at a soccer (football) game following the ball with your eyes as the swift footed players dance about the field, praying to god that someone will actually score, and suddenly a herd of cows runs onto the field.  Not likely you say?  Watch this video, and notice the crowd's reaction.  

Personally, I think they just wanted to play!