The dropdown SelectedValue is empty, even if it's loaded

 Here is the problem. There's a complicated page, which has a control. On that control I have a simple dropdown list and textbox.
The dropdown list is correctly populated in Page_Load(). Also we have a button. When I press the button, I should be redirected to another page, passing the text from the textbos and the selected value from the dropdown. But on click the textbox has value, but not the dropdown. It becomes empty and no DataSource inside.
I have realy no idea why it's happening. The code is looking like this:
 
public partial class MapSearch_WebUI_ctlLowMortgageRatesBanner : MappingControlBase
{
    private decimal _currentRate = decimal.Zero;

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
      &nbs ...