AJAX Update Panel control with dropdownlist not doing partial postback

I'm using an AJAX Extension Update panel control. I'm using 2 dropdown list controls.

here, once i select an item from the first drpdown the second dropdown should get populated with the related entries. both the dropdowns are in the update panel control. Please refer to the markup code for the same :

In the event handler for cmd_SelectedIndexChanged I have the following in code behind (refer below) :

But everytime i select an item from the cmd dropdown it does a post back refreshing the whole page. please let me know what am i doing wrong to avoid a post back & just do a partial update.

Thanks,
Kapil.

<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
    <asp:UpdatePanel Visible="true" runat="server" ChildrenAsTriggers="false" UpdateMode="Conditional">
        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="cmd" EventName="SelectedIndexChanged"/>
        </Triggers>
        <ContentTemplate>
            <asp:DropDownList ID="cmd" runat="server" AutoPostBack="true"
                onselectedindexchanged="cmd_SelectedIndexChanged">
            </asp:DropDownList>
            <asp:DropDownList ID="cmdOrders" runat="server">
            </asp:DropDownList>
        </ContentTemplate>
    </asp:UpdatePanel>

protected void cmd_SelectedIndexChanged(object sender, EventArgs e)
        {
            populateOrders(cmd.SelectedValue);
        }

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