To reproduce:
-add RadRotator
-add items to the rotator: add 3 RadLabels
-enable Running property
-run the project and you will see that initially all RadLabels are visible and they overlap each other. After all items are rotated once, the layout is correct

Workaround:
public Form1()
{
    InitializeComponent();

    this.radRotator1.Running = true;
    foreach (RadItem item in this.radRotator1.Items)
    {
        item.Visibility = ElementVisibility.Collapsed;
    } 
}