Usually in a battle, the front lines where the main fighting force clashes tends to move about. It may stay in a reasonably still position, but as one side starts to win and another starts to lose ground, the position of their fighting will change slightly, leaving the already dead bodies behind.
The difference with this fight is that they had a merciless Bolton commander, who did not care who was killed in the volleys of arrows. His archers continued to fire, wiping out both armies, in the same position. This meant when the arrows killed a majority of the force in a single position over and over, any ground that may have been gained was lost, and the sides would come together and clash in the same place.
Based on the size of the forces, it was probably artistic license that the pile got so big, enough to create a literal wall (including the fact that had the bodies fallen naturally it would likely have formed more of a hill with a shallower gradient, based on bodies simply falling down the edges).
However, I'm assuming the idea of the battle was that the unorthodox occurrence of one side firing in a single position, killing soldiers on both sides repeatedly, caused the huge pile of bodies in a single position, thus forming the wall of bodies.