gridview排序时出发RowCommand事件，为什么？

[cexbzxh]

我给GridView设置了排序，但现在一点击GridView标题，就会触发rowcommand事件，百思不得其解。
页面代码如下：

<asp:GridView ID="grdVisa" runat="server" CssClass="GridViewStyle" 
            AutoGenerateColumns="False" 
            EmptyDataText="没有可显示的数据记录。" Width="1610px" 
            onrowcommand="grdVisa_RowCommand"
            onrowdatabound="grdVisa_RowDataBound" 
            AllowSorting="True" 
            AllowPaging="True"
            PageSize="20" 
            HorizontalAlign="Center" 
            onpageindexchanging="grdVisa_PageIndexChanging" onsorting="grdVisa_Sorting" > 
            <Columns>  
                <asp:BoundField DataField="flag" HeaderText="流转状态" SortExpression="FLAG">
                    <ItemStyle Width="60px" />
                </asp:BoundField>
                <asp:BoundField DataField="sup_accept_flag" HeaderText="同意" SortExpression="sub_accept_flag">
                    <ItemStyle  Width="60px" />
                </asp:BoundField>
                <asp:ButtonField CommandName="EditVisa" Text= "编辑"
                    DataTextField="editflag" >                   
                    <ItemStyle ForeColor="#0033CC"  Width="40px" />
                </asp:ButtonField>         
                <asp:BoundField DataField="occur_time" HeaderText="日期" SortExpression="occur_time">  
                  <ItemStyle Width="100px" /> 
                </asp:BoundField>               
                <asp:BoundField DataField="visa_no" HeaderText="签证编号" SortExpression="visa_no">
                 <ItemStyle Width="100px" /> 
                </asp:BoundField>
                <asp:BoundField DataField="reason" HeaderText="原因">
                 <ItemStyle Width="100px" /> 
                </asp:BoundField>  
                <asp:BoundField DataField="contract_no" HeaderText="合同号">
                    <ItemStyle Width="200px" />
                </asp:BoundField>
                <asp:BoundField DataField="contract_name" HeaderText="合同名称" SortExpression="contract_name">
                    <ItemStyle Width="200px" />
                </asp:BoundField>              
                <asp:BoundField DataField="site_engineer_name" HeaderText="现场工程师">
                 <ItemStyle Width="100px" /> 
                </asp:BoundField>                
                <asp:BoundField DataField="builder_price_total" HeaderText="报审总价">
                 <ItemStyle Width="100px" /> 
                </asp:BoundField>
                <asp:BoundField DataField="audit_price_total" HeaderText=" 审定总价">
                 <ItemStyle Width="100px" /> 
                </asp:BoundField>  
                <asp:BoundField DataField="pm_submit_name" HeaderText="录入人" SortExpression="SUBMIT_TIME">
                    <ItemStyle Width="80px" />
                </asp:BoundField>
                <asp:BoundField DataField="submit_time" HeaderText="录入时间" SortExpression="SUBMIT_TIME">
                    <ItemStyle Width="100px" />
                </asp:BoundField>               
                <asp:BoundField DataField="visa_id" HeaderText="visa_id" />
                <asp:BoundField DataField="attach_id" HeaderText="attach_id" />
                <asp:BoundField DataField="sflag" HeaderText="sflag" />
            </Columns>
            <FooterStyle CssClass="GridViewFooterStyle" />
            <RowStyle CssClass="GridViewRowStyle" />   
            <SelectedRowStyle CssClass="GridViewSelectedRowStyle" />
            <PagerStyle CssClass="GridViewPagerStyle" />
            <AlternatingRowStyle CssClass="GridViewAlternatingRowStyle" />
            <HeaderStyle CssClass="GridViewHeaderStyle" />
        </asp:GridView>

后台排序代码：
protected void grdVisa_Sorting(object sender, GridViewSortEventArgs e)
{
if (!string.IsNullOrEmpty(e.SortExpression))
{
ViewState["SortOrder"] = e.SortExpression;
ViewState["OrderDesc"] = ViewState["OrderDesc"].ToString().Trim() == "ASC" ? "DESC" : "ASC";
LoadData();
}
}

[cexbzxh]

屏蔽掉Rowcommand事件后，就可以正常排序。 我跟踪了一下，点击列标题后，本来应该去执行OnSorting事件的，却跑到了onRowCommand事件去，好奇怪啊。