One doesn't have to understand German to get the point of this video: Horses don't like it because it hurts.
Viewing this through Google's Chrome browser may prove helpful for those who can't read German because that browser automatically brings up a toolbar for Google translate. There's not much text, but it provides some interesting points, including a comment that 70 percent of dressage riders use rollkur.
That tidbit likely explains why I've noticed an increasing number of incidents of equine rebellion in the form of balking and bolting and bucking since rollkur became fashionable. I'd never seen a top level dressage horse do any of these things in competition until the Athens Olympics. Now it's everywhere because competitive dressage horses differ from classically trained dressage horses, horses slowly coaxed and conditioned to be obedient.