asp.net dropdownlist

为什么我每次点击按纽，select中选定的项就会发生变化呢，我已经使用了Page.ispostbackprotected void databindtree()
    {
          
        try
        {
            TreeNode root = new TreeNode("知识类大纲");
            TreeView1.Nodes.Add(root);
            ds = oper_date.select_operate("select distinct zslbmc from HY0040,HY0041 where HY0040.zslbbh=HY0041.zslbbh");
            for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
            {
                TreeNode tn = new TreeNode(ds.Tables[0].Rows[i][0].ToString());
                root.ChildNodes.Add(tn);
            }
            
        }
        catch
        {
            Response.Write("<script>alert('操作数据库错误,请稍后再试!')</script>");        }
    }
    protected void databindselect()
    {
        try
        {
            
            ds = oper_date.select_operate("select zslbmc from HY0041 ");
            for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
            {
                this.Select3.Items.Add(ds.Tables[0].Rows[i][0].ToString());            }
           
        }
        catch
        {
            Response.Write("<script>alert('操作数据库错误,请稍后再试!')</script>");        }
    }
    protected void Page_Load(object sender, EventArgs e)
    {
        
        oper_date = new operatedate();
        if (!Page.IsPostBack)
        {
            this.TreeView1.Nodes.Clear();
            this.Select3.Items.Clear();
            databindtree();
            databindselect();
            
        }
            this.Button1.Disabled = true;
            this.Button2.Disabled = true;
            this.Button3.Disabled = true;
        
        
    }