, it doesn't have anything to do with topography. What it does have to do is whether or not the unit is considered a mobile armor. Apprently, ball is, so it won't be knocked down. Some suits, such as WZC, have two modes, one in MS and another in snipe, when it's in snipe mode it's considered a mobile armor and can't be knocked down.
Also, another thing about mobile armors, is that they don't flinch, that is when you hit them with a melee attack. They don't get stun by it and can punch back, that's what makes it a bad idea to melee MA's from the front, eg.,, Psycho Gundam.