求高手解决一个很灵异的问题～～急～～～
我用UpdatePanel空间写了一个绑定DropDownList的功能，要求绑定 市 区 商圈，绑定没问题 运行没问题，但是点保存按钮的时候，DropDownList的SelectedIndexChanged事件会重新运行一遍，最后取到的结果是请选择，高手帮我看看咋回事，下面是代码。

HTML代码

<%@ Page Language="C#" MasterPageFile="~/Main.Master" EnableEventValidation="false" ResponseEncoding="GB2312" AutoEventWireup="true" CodeFile="PurchaseEdit.aspx.cs" Inherits="Personal_PurchaseEdit" Title="发布求购信息"%>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<script type="text/javascript" src="../js/calendar.js"></script>
<table width="100%" border="0" cellspacing="0" cellpadding="0">
  <tr>
  <td class="t_link"><ul><li>
  <asp:Label ID="Label10" runat="server" Text="所在位置"></asp:Label>:
  <a href="../Default.aspx" ><asp:Label ID="Label11" runat="server" Text="首 页" ></asp:Label></a> >> 
  <asp:Label ID="Label9" runat="server" Text="发布求购信息" ></asp:Label></li></ul></td>
  </tr>
  </table>
  <table border="0" cellpadding="0" cellspacing="0" class="hmain00" align="center">
  <tr>
  <td class="hmain01_1">
  <ul class="ttop02"><li></li></ul>
  <ul class="hmain04">
  <li style="width:358px;">
  <asp:Label ID="Label1" runat="server" Text="地 区:" CssClass="hmtext80"></asp:Label>
   
  <asp:ScriptManager ID="ScriptManager1" runat="server">
  </asp:ScriptManager><asp:UpdatePanel id="UpdatePanel1" runat="server"><contenttemplate>
<asp:DropDownList id="ddlCity" runat="server" Width="70px" OnSelectedIndexChanged="ddlCity_SelectedIndexChanged" AutoPostBack="True">
  </asp:DropDownList> <asp:DropDownList id="ddlTown" runat="server" Width="70px" AutoPostBack="True" OnSelectedIndexChanged="ddlTown_SelectedIndexChanged">
  </asp:DropDownList> 
  <asp:DropDownList ID="ddlArea" runat="server" Width="70px">
  </asp:DropDownList>
  <span style="color: #ff0000">*</span>
</contenttemplate>
  </asp:UpdatePanel>
<li style="width:475px;">
  <asp:Label ID="Label4" runat="server" Text="关键字:" CssClass="hmtext80"></asp:Label>
  <asp:TextBox ID="KeyWord1" runat="server"></asp:TextBox>
  <asp:TextBox ID="KeyWord2"
  runat="server"></asp:TextBox>
  <asp:TextBox ID="KeyWord3" runat="server"></asp:TextBox><span style="color: #ff0000">*</span></li></ul>
  <ul clas