回发事件

问题如下：
.aspx代码                    <div class="divstyle1">
                    <asp:Label ID="lablenewscount" runat="server"></asp:Label>
                    <asp:LinkButton ID="linkfirst" runat="server" Text="首页" OnClick="IndexChanging" CommandArgument="first"></asp:LinkButton>
                    <asp:LinkButton ID="linkpre" runat="server" Text="上一页" OnClick="IndexChanging" CommandArgument="pre"></asp:LinkButton>
                    <asp:LinkButton ID="linknext" runat="server" Text="下一页" OnClick="IndexChanging" CommandArgument="next"></asp:LinkButton>
                    <asp:LinkButton ID="linklast" runat="server" Text="尾页" OnClick="IndexChanging" CommandArgument="last"></asp:LinkButton>
                    <asp:Label ID="labelpagemsg" runat="server"></asp:Label>
                    <asp:DropDownList ID="droppage" runat="server" Height="20px" 
                        AutoPostBack="True" onselectedindexchanged="droppage_SelectedIndexChanged" CssClass="buttontype2"></asp:DropDownList>
                    </div>
.cs代码    protected void IndexChanging(object sender, EventArgs e)
    {
        string strCommand = ((LinkButton)sender).CommandArgument.ToString();
        //
    }我在IndexChanging，和Page_Load这两处设置断点，当第一次按下一页时，探测到页面回发，第二次按任何按钮都没探测到页面回发，也没有报错。