Had the Carthaginian cavalry effectively defeated the Roman cavalry, the Carthaginian infantry was doing the same with the Roman legions. The Roman infantry had continued to drive forward, and had driven itself into an alley formed by the light Carthaginian infantry stationed at the sides.
Shielded by these Carthaginian troops, their comrades who had stayed at the rear could now swing around and come in behind the Roman army. The Roman doomed legions were encircled and being attacked from all sides.
In effect the Roman infantry had been defeated by the opposing infantry, although the returning Carthaginian cavalry helped further accelerate their victory.