Brothers In Code

I had a combo box was being displayed based on the the value of another control.  To call attention to it's appearance, I wanted it to "fly in" by animating its size.  Below is my WPF style.  There are a couple of things that I couldn't figure out.  First, having it "fly away" in the same fashion is out since it disappears before any animation takes place (although I have seen people do it in code behind).  Second, I'm not quite sure why I need the first setter value.  I seems like the one in the trigger should work but by itself, it wasn't enough.

    <Style TargetType="{x:Type ComboBox}" >
      <Setter Property="RenderTransform">
        <Setter.Value>
          <ScaleTransform ScaleX="0" ScaleY="0"/>
        </Setter.Value>
      </Setter>
      <Style.Triggers>
        <Trigger Property="Visibility" Value="Visible">
          <Trigger.EnterActions>
            <BeginStoryboard>
              <Storyboard>
                <DoubleAnimation Storyboard.TargetProperty="RenderTransform.ScaleX" To="1" Duration="00:00:2">
                  <DoubleAnimation.EasingFunction>
                    <PowerEase Power="3" EasingMode="EaseInOut"/>
                  </DoubleAnimation.EasingFunction>
                </DoubleAnimation>
                <DoubleAnimation Storyboard.TargetProperty="RenderTransform.ScaleY" To="1" Duration="00:00:2">
                  <DoubleAnimation.EasingFunction>
                    <PowerEase Power="3" EasingMode="EaseInOut"/>
                  </DoubleAnimation.EasingFunction>
                </DoubleAnimation>
              </Storyboard>
            </BeginStoryboard>
          </Trigger.EnterActions>
        </Trigger>
        <Trigger Property="Visibility" Value="Hidden">
          <Setter Property="RenderTransform">
            <Setter.Value>
              <ScaleTransform ScaleX="0" ScaleY="0"/>
            </Setter.Value>
          </Setter>
        </Trigger>
      </Style.Triggers>
    </Style>