Being charged from a direction you cannot see and thought was safe is a frightening thing for even the bravest of troops. With his field of vision narrowed by his position in the ranks, the average soldier has only a limited knowledge of what is happening on the battlefield. If the enemy has somehow got round behind him he might reasonably assume that the battle is lost. Doubt will be sown in his mind and he may become disoriented and confused. His fighting efficiency may even become impaired. If he is fighting in close combat, he might panic and flee, thinking that the enemy is upon his unguarded back.
Individual models can turn to face the chargers and avoid this test and any other penalties, unless they are already engaged in close combat.