Orcs expect the smaller greenskins to run away and it doesn't really surprise them when they do. The sight of fleeing Goblins and Snotlings simply reminds the bigger and bolder Orcs why they are best. Consequently when a Goblin unit, Snotling Swarm or Snotling Pump Wagon is destroyed, breaks or flees past, Orc units do not need to test for panic.