When dealing with literally hundreds of tons of moving beasts while sitting on the back of another moving beast, something at some point is bound to go wrong.
Maybe your horse bucks when you're not ready and you go flying fifteen feet, breaking the ground with your tailbone and the back of your skull. Maybe you catch a hoof in the wrong area (is there a right area to catch a hoof?) while attempting to hog-tie a calf at the annual county rodeo. And let's not forget everybody's favorite livestock-related disaster, the stampede.
At any rate, working with animals requires you to know what you're doing, and to be ready to go with a bag of ice and a bottle of ibuprofen should the situation require them.