I am not understanding why this doesn't fire. It will not even call the event. Any thoughts? Really what I want to do, is prevent a full post-back. The page is large and each time it does a post back it jumps to the top.

<asp:UpdatePanel runat="server" id="UpdatePanel" updatemode="Conditional">
    <ContentTemplate>
    <p>
        <asp:Label runat="server" ID="lblSnippet" AssociatedControlID="cmbSnippet">
            From snippet:
        </asp:Label>
        <asp:DropDownList runat="server" ID="cmbSnippet" CssClass="dropdown" AutoPostBack="true">
            <asp:ListItem>
                None
            </asp:ListItem>
            <asp:ListItem>
                Read Part
            </asp:ListItem>
            <asp:ListItem>
                Read Feed
            </asp:ListItem>
        </asp:DropDownList>
    </p>
    <!--Code-->
    <p>   
        <asp:Label runat="server" ID="lblCode" AssociatedControlID="txtCode">
            From code:
        </asp:Label>
        <asp:TextBox runat="server" ID="txtCode" TextMode="MultiLine" CssClass="editor" MaxLength="100" Height="200" Wrap="false" />
    </p> 
    </ContentTemplate>
    <Triggers>
         <asp:Asyncpostbacktrigger controlid="cmbSnippet" eventname="SelectedIndexChanged" />
    </Triggers>
</asp:UpdatePanel>