It might because of your trigger ControlID is out of scope to Trigger collection.Also
You have set
lnkbtncommit
as the control ID for the update panel. Try setting it like this,
trigger.ControlID = "DropDownList1";
trigger.EventName = "SelectedIndexChanged";
Also you can set your dropdownlist ID for the update panel trigger like below in aspx
<asp:updatepanel id="UpdatePanel1" runat="server">
<triggers>
<asp:asyncpostbacktrigger controlid="DropDownList1" eventname="SelectedIndexChanged" />
</triggers>
<contenttemplate>
.....</contenttemplate></asp:updatepanel>