To reproduce:

public partial class Form2 : RadRibbonForm
{
    public Form2()
    {
        InitializeComponent(); 

        this.WindowState = FormWindowState.Maximized;
        this.radRibbonBar1.ApplicationMenuStyle = ApplicationMenuStyle.BackstageView;
        this.AllowAero = false;         
        this.Icon = Properties.Resources.WinFormsIcon;     
    }
}

Workaround1: set AllowAero property to true if it is possible.
Workaround2: this.radRibbonBar1.RibbonBarElement.RibbonCaption.MouseMove += RibbonCaption_MouseMove;
private void RibbonCaption_MouseMove(object sender, MouseEventArgs e)
{
    if (e.Button == System.Windows.Forms.MouseButtons.Left && this.WindowState== FormWindowState.Maximized)
    {
        this.WindowState = FormWindowState.Normal;
        this.Location = new Point(this.Location.X,e.Location.Y);
    }
}