How is that naked dude stopping the horse in your gif?
Well it's not that accurate of what it's trying to represent. It should really be the horse stopping, and then trying to move but one person is blocking the horse from moving forward.
I think of it as a combination of two issues, horses can be stopped, or slowed to an almost full stop, while riding at speed if two people are right next to each other (or if you hit a slight terrain change before going over the person).
The other issue (which the pic was supposed to represent) is that when a horse stops, if someone stands in front of it, they can completely prevent the horse from going forward. Which AFAIK has always been that way in Warband/crpg, but it's pretty annoying (I kept a donkey rider teammate stuck in the open field by keeping in front of the donkey's head when he tried to turn and go around me). If a horse gets boxed into a corner they are effectively stuck there if someone stands in front of it.