<Window x:Class="TreeViewSelection.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <TreeView Name="TV">
            <TreeViewItem Header="Привет-привет"/>
            <TreeViewItem Header="Пока-пока"/>
            <TreeViewItem>
                <TreeViewItem Header="Я очень"/>
                <TreeViewItem Header="буду ждать">
                    <TreeViewItem Header="звонка"/>
                </TreeViewItem>
            </TreeViewItem>
        </TreeView>
        <TreeView Name="TV2" Grid.Row="1" ItemsSource="{Binding}">
            <TreeView.ItemTemplate>
                <HierarchicalDataTemplate ItemsSource="{Binding Value}">
                    <TextBlock Text="{Binding Key}"/>
                </HierarchicalDataTemplate>
            </TreeView.ItemTemplate>
        </TreeView>
    </Grid>
</Window>

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        DataContext = new[]
                      {
                          new DataHolder("Привет-привет", null),
                          new DataHolder("Пока-пока", null),
                          new DataHolder(null,
                              new[]
                              {
                                  new DataHolder("Я очень", null),
                                  new DataHolder("буду ждать",
                                      new[]
                                      {
                                          new DataHolder("звонка", null)
                                      }),
                              }),
                      };

        TreeViewItem selectedItemTV = null;
        TV.AddHandler(TreeViewItem.SelectedEvent, (RoutedEventHandler)((sender, args) =>
                    selectedItemTV = (TreeViewItem)args.OriginalSource));

        TreeViewItem selectedItemTV2 = null;
        TV2.AddHandler(TreeViewItem.SelectedEvent, (RoutedEventHandler)((sender, args) =>
                    selectedItemTV2 = (TreeViewItem)args.OriginalSource));

        var dt = new DispatcherTimer() { Interval = TimeSpan.FromSeconds(5) };
        dt.Tick += (sender, args) =>
            {
                if (selectedItemTV != null)
                {
                    selectedItemTV.IsSelected = false;
                    selectedItemTV = null;
                }
                if (selectedItemTV2 != null)
                {
                    selectedItemTV2.IsSelected = false;
                    selectedItemTV2 = null;
                }
            };
        dt.Start();
    }
}

public class DataHolder
{
    public string Key { get; private set; }
    public IEnumerable<DataHolder> Value { get; private set; }

    public DataHolder(string key, IEnumerable<DataHolder> value)
    {
        Key = key;
        Value = value;
    }
}

((TreeViewItem)treeView.SelectedItem).IsSelected = false;