Because McMMOPlayerUnarmEvent is a silly name.

Changelog.txt

@@ -11,6 +11,7 @@ Version 1.4.00-dev
 
 Version 1.3.14
  + Added new Hylian Luck skill to Herbalism.
+ + Added new cancellable McMMOPlayerDisarmEvent for Citizens compatibility - fires whenever a player is disarmed.
  = Fixed a memory leak involving mob tracking
  - Removed extra durability loss from Leaf Blower
 

src/main/java/com/gmail/nossr50/events/skills/McMMOPlayerUnarmEvent.java → src/main/java/com/gmail/nossr50/events/skills/McMMOPlayerDisarmEvent.java

@@ -5,11 +5,11 @@ import org.bukkit.event.Cancellable;
 
 import com.gmail.nossr50.datatypes.SkillType;
 
-public class McMMOPlayerUnarmEvent extends McMMOPlayerSkillEvent implements Cancellable {
+public class McMMOPlayerDisarmEvent extends McMMOPlayerSkillEvent implements Cancellable {
     private boolean cancelled;
     private Player defender;
 
-    public McMMOPlayerUnarmEvent(Player defender) {
+    public McMMOPlayerDisarmEvent(Player defender) {
         super(defender, SkillType.UNARMED);
         this.defender = defender;
     }

src/main/java/com/gmail/nossr50/skills/unarmed/DisarmEventHandler.java

@@ -5,7 +5,7 @@ import org.bukkit.entity.Player;
 import org.bukkit.inventory.ItemStack;
 
 import com.gmail.nossr50.mcMMO;
-import com.gmail.nossr50.events.skills.McMMOPlayerUnarmEvent;
+import com.gmail.nossr50.events.skills.McMMOPlayerDisarmEvent;
 import com.gmail.nossr50.locale.LocaleLoader;
 import com.gmail.nossr50.util.Misc;
 
@@ -36,7 +36,7 @@ public class DisarmEventHandler {
     }
 
     protected void handleDisarm() {
-        McMMOPlayerUnarmEvent event = new McMMOPlayerUnarmEvent(defender);
+        McMMOPlayerDisarmEvent event = new McMMOPlayerDisarmEvent(defender);
         mcMMO.p.getServer().getPluginManager().callEvent(event);
 
         if(!event.isCancelled()) {