SPGridViewPager Calling Full Page Postback in UpdatePanel

I have a SPGridView and a SPGridViewPager control inside an update panel in a web part.  The gridview posts asynchronously, but the pager calls a full page postback.

Why isn't the pager calling an asynchronous post back?

Code :

<asp:UpdatePanel ID="UpPanLayout" runat="server" UpdateMode="Conditional" ChildrenAsTriggers="true">

    <ContentTemplate>

    <SharePoint:SPGridView ID="tblHist" runat="server" EnableViewState="true"
                           OnRowCreated="tblHist_RowCreated" OnRowDataBound="tblHist_RowDataBound"
                           AutoGenerateColumns="false">                               
    </SharePoint:SPGridView>

    <SharePoint:SPGridViewPager ID="pager" GridViewId="tblHist" runat="server" EnableViewState="true" OnClickNext="pager_PageChange" OnClickPrevious="pager_PageChange"></SharePoint:SPGridViewPager>

    </ContentTemplate>
</asp:UpdatePanel>