Also, my feeling is that Lord Arad might have hired some barbarians to stiffen up the infantry. So tougher melee fighters to help support the line. They're still cavalry heavy and facing a warg cavalry that has serious advantages against bunny heavy cavalry. 

The human mages are probably softening up the ground to bog down the orcish infantry, as well, as they are heavily armored and this would give advantage to the barbarian infantry.