Asp.net Controlls acting weird

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ASP.NET

DropDownList

Hello All,
I have a dropdown in my aspx page from which I am downloading data.
From dropdown I will choose the file format and data will be downloaded in that particular format

Here is my drop-down.

<asp:DropDownList ID="ddlExportRprts" runat="server" AutoPostBack="true"  OnSelectedIndexChanged="ddlExportRprts_SelectedIndexChanged">
    <asp:ListItem Text="Select" Value="0" Selected="True"></asp:ListItem>
    <asp:ListItem Text="Excel" Value="1"></asp:ListItem>
    <asp:ListItem Text="CSV" Value="2"></asp:ListItem>
</asp:DropDownList>

and here is the back-end method of the dropdown.

protected void ddlExportRprts_SelectedIndexChanged(object sender, EventArgs e)
{
 string strFileName="Test";
 string strDataFilePath = Server.MapPath("./AttachedFiles/") + "\\" + strFileName + ".xls";
 DownLoadFile(strDataFilePath, strFileName);
}

private void DownLoadFile(string strFilePath, string strFileName)
{
  if (System.IO.File.Exists(strFilePath))
  {
      Response.ContentType = "application/octet-stream";
      Response.AppendHeader("Content-Disposition", "attachment; filename =" + strFileName + ".xls");
      Response.TransmitFile(strFilePath);
      Response.End();
  }
}

when ever I am selecting any option from drop down file is getting downloaded in selected format..no issues with that.

but after file is getting download and then I m clicking on any server side control like button, link button, check-box, radio-button any other drop down, this method
"ddlExportRprts_SelectedIndexChanged()" is being called where as these controls has there on click events.

I am confuse why these method is being fired when I m not calling it.

I am stuck in this problem from long time, I hope some1 out there knew how to solve it..

cheers in advance..
: )