full post back occurs

i have a grid in

inspite of it whenver a postback occurs there is full post back

<asp:UpdatePanel ID="UpdatePanel2" runat="server">
                 <ContentTemplate>


grid

       <asp:Button runat="server" ID="posyback" OnClick="posyback_Click" Text="" />
                 </ContentTemplate>
                 <Triggers>
                     <asp:PostBackTrigger ControlID="posyback" />
                 </Triggers>
             </asp:UpdatePanel>

when grid header is clicked  it initates posyback_Click but full post backcan be visible with  lot of time for loading the grid