In your codebehind,
public DockWindow()
{
InitializeComponent();
this.Left = this.Width + SystemParameters.FullPrimaryScreenWidth;
}
Change your trigger to
<Window.Triggers>
<EventTrigger RoutedEvent="Window.Loaded">
<BeginStoryboard>
<Storyboard >
<DoubleAnimation Duration="0:0:1.5" Storyboard.TargetProperty="Left" To="10" AccelerationRatio=".1"/>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Window.Triggers>