Minecraft wiki entry for Monster_infighting, associated text: If a hostile mob is directly attacked by a player or another mob, the damaged mob stops attacking its current target and attacks the mob or player that damaged it. This causes the mob that did the damage to retaliate in turn, and the two of them become locked into a duel until one of them dies. This is useful in combat as it distracts two monsters temporarily, kills one of them, and severely weakens the other.