use a fake/dummy button as the target control for your modal popup. Just set style="display:none" so it will become invisible. For example:
<asp:linkbutton id="lnkTarget" runat="server" style="displaye:none;" xmlns:asp="#unknown" />
<asp:panel id="pnlPopUp" runat="server" style="display:none" cssclass="modal" xmlns:asp="#unknown">
Your content
</asp:panel>
<asp:modalpopupextender id="ModalPopupExtender1" runat="server" targetcontrolid="lnkTarget" popupcontrolid="pnlPopUp" backgroundcssclass="modal-bg" xmlns:asp="#unknown">
</asp:modalpopupextender>
Then call:
ModalPopUpExtender1.Show();
In the event where you show the modal, on paging and on sorting event.