DataTrigger打破了前一个

Question

有两个DataTriggers。启动应用程序后，两个触发器都工作，但在第二个触发器触发后，第一个触发器不再工作。如何解决这个问题？

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Window.Resources>
    <Style TargetType="{x:Type TextBlock}">
        <Style.Triggers>
            <DataTrigger Binding="{Binding ElementName=ButDown,Path=IsPressed}" Value="True">
                <DataTrigger.EnterActions>
                    <BeginStoryboard>
                        <Storyboard>
                            <ThicknessAnimation Storyboard.TargetProperty="Margin" By="20" />
                        </Storyboard>
                    </BeginStoryboard>
                </DataTrigger.EnterActions>
            </DataTrigger>                
            <DataTrigger Binding="{Binding ElementName=ButUp,Path=IsPressed}" Value="True">
                <DataTrigger.EnterActions>
                    <BeginStoryboard>
                        <Storyboard>
                            <ThicknessAnimation Storyboard.TargetProperty="Margin" By="-20" />
                        </Storyboard>
                    </BeginStoryboard>
                </DataTrigger.EnterActions>
            </DataTrigger>
        </Style.Triggers>
    </Style>
</Window.Resources>
<StackPanel>
    <Button x:Name="ButUp">UP</Button>
    <Button x:Name="ButDown">DOWN</Button>
    <TextBlock Margin="100">TextBlock</TextBlock>
</StackPanel>
</Window>