Put your datagrid in the update panel, and register the pageindexchanged event to the trigger. or register it wwhile in the onrowdatabound event. it is working.
your page will not be postback,on pageindexchanged event.
put your code in the content
<asp:UpdatePanel ID="pnlupdate", runat="server" >
<ContentTemplate>
</ContentTemplate>
</asp:UpdatePanel>