Modal Popup Extender and refreshing page

Hello!

When i use TargetControlID in Modal Popup Extender, my modal popup displayed on page without refreshing parent page. But when i use modal server side function modalpoupextender.Show(), my parent page whole refreshing, before that what displayed popup. Why it's happens?

content.aspx:

<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager><asp:Panel ID="modalOrder1" runat="server" CssClass="modalPopupEtp" style="display:none; background-color:#FFF; width:505px; z-index:10001 !important;"><asp:LinkButton ID="CloseOrder" runat="server" onclick="CloseOrder_Click" CssClass="modal_close" Text="Закрыть"></asp:LinkButton><div class="clear"></div><asp:Label ID="priceCert" runat="server" style="padding-left:90px; color:#095353; font-size:15px;"></asp:Label><br /><br /></asp:Panel><ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender1" runat="server"
    TargetControlID="btnDummy" PopupControlID="modalOrder1" BackgroundCssClass="modalBackground"
    DropShadow="true" CancelControlID="CloseOrder"></ajaxToolkit:ModalPopupExtender><asp:Button ID="btnDummy" runat="server" style="display:none;"/><asp:LinkButton ID="orderUniversal1" runat="server" NavigateUrl="#" ToolTip="Заказать" OnClick="MakeOrder_Click"><img src="../Images/green_order_button.png"  style="position:relative; margin:0 auto; border:0;" /></asp:LinkButton>

code behind

protected void MakeOrder_Click(object sender, EventArgs e)
    {
        priceCert.Text = "0000";
        ModalPopupExtender1.Show();
    }

Thanks!