问题与Jquery Ajax和页面加载会话分配

我有一个asp.net后台工作者。我想通过的百分比计数的进展改变，并显示在jquery的进度条。密码就像魔法一样有效进度条百分比显示…0% -> 10% -> 20% -> ....——>

100%

但不幸的是，当我在页面加载中分配Session["UserName"]="abc"时，它不会执行异步回发!进度条百分比显示…0%——(几秒钟后直接)——> 100%

Jquery

$(document).ready(function() {
            $("#progressbar").progressbar();
                  setTimeout(updateProgress, 100);
        });
       function updateProgress() {
                  $.ajax({
                      type: "POST",
                      url: "Downloader.aspx/GetData",
                      data: "{}",
                      contentType: "application/json; charset=utf-8",
                      dataType: "json",
                      async: true,
                      success: function(msg) {
                          // Replace the div's content with the page method's return.
                          $("#progressbar").progressbar("option", "value", msg.d);
                      }
                  });
              }

Downloader.aspx.cs

static BackgroundWorker _bw;
public static int Percent { get; set; }
    protected void Button1_Click(object sender, EventArgs e)
    {
        _bw = new BackgroundWorker
        {
            WorkerReportsProgress = true,
            WorkerSupportsCancellation = true
        };
        _bw.DoWork += bw_DoWork;
        _bw.ProgressChanged += bw_ProgressChanged;
        _bw.RunWorkerCompleted += bw_RunWorkerCompleted;
        _bw.RunWorkerAsync("Hello world");
    }
    void bw_DoWork(object sender, DoWorkEventArgs e)
    {
        for (int i = 0; i <= 100; i += 20)
        {
            if (_bw.CancellationPending) { e.Cancel = true; return; }
            _bw.ReportProgress(i);
            Thread.Sleep(1000);
        }
        e.Result = 123;
    }
    void bw_RunWorkerCompleted(object sender,
                                       RunWorkerCompletedEventArgs e)
    {
            percentage.Text = "Complete: " + e.Result;      // from DoWork
    }
    void bw_ProgressChanged(object sender,
                                    ProgressChangedEventArgs e)
    {
        Percent = e.ProgressPercentage;
    }
    [WebMethod]
    public static int GetData()
    {
        return Percent;
    }

上面的代码是工作的，直到我添加Session["UserName"]="abcd"在页面加载

Downloader.aspx.cs

   protected void Page_Load(object sender, EventArgs e)
    {  
        Session["userName"] = "abcd!";
    }

谁能告诉我发生了什么事?我已经花了2天时间想明白了，但还是不明白。