I thought the LOTR fans might be interested in this.
Jackson's mistake? The modeling software was "Made In France".
The extended Two Towers "making of" said the problem was that the CGI "agents" representing individual soldiers were programmed to run in the direction they were facing when they first appeared, in search of enemies to fight, if they could not initially see any enemies. And the ones who appeared in the rear ranks couldn't see any enemies. Those whose first appearance had them facing to the rear ran in that direction. Those in the rear ranks who faced to the flank ran to the flanks until they could see enemies. Those who faced to the front couldn't move forward, and so didn't move at all until the whole line shuffled forward.
I imagine they solved the problem by having the "agents" first appearance have them all facing in the same direction.