When you're walking, your velocity is constant, so the average of the horizontal forces over time cancel. You push forwards with your front foot and backwards with the back foot. As you place your front foot down, it's a dynamic situation, and more likely to slip, than the back foot, so the front foot ends up sliding forwards.
Also if a person is just standing, the pivot point is at the back of the foot allowing a person to lean forwards a bit but still keep the center of gravity over the foots contact patch. However the person can't lean back because there very little foot contact patch behind the ankle. This would make it more likely for a person to fall backwards.