So I've been told what I'm doing here is wrong, but I'm not sure why.

I have a webpage that imports a CSV file with document numbers to perform an expensive operation on. I've put the expensive operation into a background thread to prevent it from blocking the application. Here's what I have in a nutshell.

protected void ButtonUpload_Click(object sender, EventArgs e)
{
    if (FileUploadCSV.HasFile)
    {
        string fileText;
        using (var sr = new StreamReader(FileUploadCSV.FileContent))
        {
            fileText = sr.ReadToEnd();
        }

        var documentNumbers = fileText.Split(new[] {',', '\n', '\r'}, StringSplitOptions.RemoveEmptyEntries);

        ThreadStart threadStart = () => AnotherClass.ExpensiveOperation(documentNumbers);
        var thread = new Thread(threadStart) {IsBackground = true};
        thread.Start();
    }
}

(obviously with some error checking & messages for users thrown in)

So my three-fold question is: a) Is this a bad idea? b) Why is this a bad idea? c) What would you do instead?