update panel control

hi

I use update panel for my dropdownlist

               <asp:UpdatePanel ID="uptran" runat="server">
                                  <ContentTemplate>
        <span style="color:Red">*</span> city:&nbsp  <asp:DropDownList ID="ddlTran" runat="server" CssClass="ddlT">
        </asp:DropDownList>
          </ContentTemplate>
                </asp:UpdatePanel>

   <asp:UpdatePanel ID="Upbdt2" runat="server">
    
    <ContentTemplate>
    <asp:ModalPopupExtender DropShadow="true" ID="ModalPopupExtender2"  
            PopupControlID="PnPopup2" runat="server" TargetControlID="BtnPopup2" 
             >
    </asp:ModalPopupExtender>
    <asp:Button ID="BtnPopup2" OnClick="BtnPopup2_Click" runat="server" Text="select image" CssClass="DPC9_Img1M" BorderStyle="Solid" BorderWidth="0px" BorderColor="White"  CausesValidation = "false"/>
    <asp:Panel ID="PnPopup2" runat="server" BackColor="White"  
            CssClass="panel">
        <asp:ImageButton ID="ImageButton4" runat="server" ImageUrl="~/image/close.png"  CausesValidation="false"/>
   <br />
        <asp:FileUpload ID="fup3" runat="server" 
            ToolTip="click for upload" CssClass="fup" />
 
  <asp:ImageButton ID="BtnUpload2" OnClick="BtnUpload2_Click"  runat="server"   CausesValidation="false" ImageUrl="~/image/load.png"  CssClass="load"/>
    
    </asp:Panel>
    </ContentTemplate>   
    <Triggers>
    <asp:PostBackTrigger ControlID="BtnUpload2" />
    
    </Triggers>
    
    </asp:UpdatePanel>

in above code when users click on (select image ) Text it open popup and they can upload their image from fileupload control

now my problem is:

when i select item from DropDownList it show popup menu that is in above code

why this happen?

thanks alot