using updatepanel and enabling texbox if ispostback

Hi. I have only a TextBox and a DropDownlist on aspx-file. TextBox enabled property is false by default. Texbox is in the UpdatePanel. And also I'm using MasterPage, but TextBox and DropDownlist aren't on the MasterPage. 

 protected void Page_Load(object sender, EventArgs e)
    {

        if (IsPostBack && DropDownList1.SelectedIndex != 0)
        {
            tbAuto.Enabled = true;
            Button2.Enabled = true;
        }



    }

Using this code. I'm not only getting a textbox with enabled true, but also the same textbox with enabled false which was by default.

How to solve it?

Can you please also explain me, why is it so?