mistakenly had DefaultStyleKeyProperty.OverrideMetadata(typeof(BusyIndicator), new FrameworkPropertyMetadata(typeof(BusyIndicator))); in a puclic constructor. Moved it to a static constructor. placed _displayAfterTimer.Tick event hookup in the overridden OnIntialize method.

ExtendedWPFToolkitSolution/Src/WPFToolkit.Extended/BusyIndicator/BusyIndicator.cs

@@ -29,16 +29,21 @@ namespace Microsoft.Windows.Controls
 
         #region Constructors
 
-        public BusyIndicator()
+        static BusyIndicator()
         {
             DefaultStyleKeyProperty.OverrideMetadata(typeof(BusyIndicator), new FrameworkPropertyMetadata(typeof(BusyIndicator)));
-           _displayAfterTimer.Tick += new EventHandler(DisplayAfterTimerElapsed);
         }
 
         #endregion //Constructors
 
         #region Base Class Overrides
 
+        protected override void OnInitialized(EventArgs e)
+        {
+            base.OnInitialized(e);
+            _displayAfterTimer.Tick += new EventHandler(DisplayAfterTimerElapsed);
+        }
+
         /// <summary>
         /// Overrides the OnApplyTemplate method.
         /// </summary>