If you set the RadNavigationView's IsPaneOpen property in XAML and start the application, the items are is not opened (the pane is collapsed). You can notice that IsPaneOpen property is set back to false, when the control gets loaded.
To work this around, subscribe to the Loaded event of the RadNavigationView instance and in the event handler, set the IsPaneOpen property.
private void RadNavigationView_Loaded(object sender, RoutedEventArgs e)
this.radNavigationView.IsPaneOpen = true;