Some horses do end up rearing to get out of work if the situation is handled in a certain manner. Sure, it never starts out that way. I agree with kevinshorses that horses rear the first time 100% out of panic or confusion. BUT, if the person riding them gets off immediately and walks them back to the barn because they're scared to get back on - the horse may associate rearing with getting to go back to the barn and not having to work, and then it becomes a habit. Most likely that confusion is still there, but at a certain point the rearing may not even be triggered by a confusing cue from the rider anymore. I've known a few horses that were like this. But yes, most horses who rear are confused or panicked.