WPF event mystery

I am new in WPF and I am unable to understand this behavior of my code... I have a class of user control...

public class CustomControl1 : Control
{
    .... some code here ... 

    /// <summary>
    /// Constructor
    /// </summary>
    public CustomControl1()
    {
        // user items
        UserItems = new ObservableCollection<Label>();
        UserItems.CollectionChanged += UserItems_CollectionChanged;            
    }

    // informace o pridani do kolekce
    void UserItems_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        // !! ?? WHY IS THIS EVENT FIRED AFTER XAML SETS UserItems PROPERTY TO NEW INSTANCE ??  
    }

    /// <summary>
    /// User items
    /// </summary>
    public ObservableCollection<Label> UserItems
    {
        get { return (ObservableCollection<Label>)GetValue(UserItemsProperty); }
        set 
        {
            // XAML code sets this value with new instance of ObservableCollection !
            SetValue(UserItemsProperty, value);
        }
    }

    // Using a DependencyProperty as the backing store for UserItems.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty UserItemsProperty =
        DependencyProperty.Register("UserItems", typeof(ObservableCollection<Label>), typeof(CustomControl1), new UIPropertyMetadata(null));

}

I use this custom control in XAML window...

<Window
        xmlns="http://ift.tt/o66D3f"
        xmlns:x="http://ift.tt/mPTqtT"
        xmlns:c="clr-namespace:WpfCustomControlLibrary1;assembly=WpfCustomControlLibrary1" x:Class="WpfApplication1.MainWindow"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <c:CustomControl1>
            <c:CustomControl1.UserItems>
                <Label>Menu1</Label>
                <Label>Menu2</Label>
            </c:CustomControl1.UserItems>
        </c:CustomControl1>
    </Grid>
</Window>

When I debug this code,

1) Constructor creates new instance of ObservableCollection and registers UserItems_CollectionChanged. to this instance.

2) UserItems property setter is raised and UserItems is set to a NEW INSTANCE of ObservableCollection. This instance should not have UserItems_CollectionChanged registered to it.

3) Event UserItems_CollectionChanged is fired for Menu1 and Menu2 labels. WHY? new instance of ObservableCollection should not raise this events !

There is something in this code or behind WPF what I do not understand. Can you help me ? Sorry for my bad English.