To reproduce :

this.radCollapsiblePanel1.IsExpanded = false;
for (int i = 0; i < 5; i++)
{
    RadButton btn = new RadButton();
    btn.Text = "Left" + i;
    btn.Location = new Point(10, 45 * i);
    this.radCollapsiblePanel1.Controls.Add(btn);

    RadButton btn2 = new RadButton();
    btn2.Text = "Right" + i;
    btn2.Location = new Point(140, 45 * i);
    this.radCollapsiblePanel1.Controls.Add(btn2);
}

Workaround:
this.radCollapsiblePanel1.IsExpanded = false; 
this.radCollapsiblePanel1.ControlsContainer.AutoScroll = true;