It's not just shields...it's just blocking from horseback in general.
many times I've been directly behind a lancer (on my lancer) stab him in the back with my lance, and it's blocked because he's holding down block on the lance. Same thing if I go to stab the horse (aiming very low) and I get blocked cleanly by the melee weapon (not a shield) and don't get a hit on the horse.