They are very, very heavy. And they are not very well aware of their surroundings.
So Heaven help you if you are standing behind them, and they decide to move, and you aren't as fast as you should be.
Why do I say this?
I say this because that happened to me last week. And the weight differential between said cow and me is about 800 or 900 pounds in their favor. And now my foot is in a lot of pain after having a cow stand on top of it for even a few seconds.
Let it be a lesson: when you're dealing with cows, or any creature that is several hundred pounds heavier than you are and fairly stupid, you're the one who has to be quick on your toes. Or else you probably won't be quick on your toes for at least a few days.