They probably enabled friendly horse bumps by putting all cavalry players into a different (virtual, so to speak) team on spawning, so VV thinks they are enemies.
Could be tested by spawning on foot, then getting a horse and trying to bump a teammate. Probably not gonna work.
edit: nope, can bump anyway...
edit2: but when I spawn as cav, sometimes VV doesn't attack, no idea
edit3: after VV has been kicked, she still attacks you, but can't hit you anymore
maybe this helps in debugging
