To reproduce:
public Form1()
{
    InitializeComponent();
    this.radListView1.ViewType = Telerik.WinControls.UI.ListViewType.IconsView;
}

private void Form1_Load(object sender, EventArgs e)
{   
    radListView1.ListViewElement.ViewElement.Orientation = Orientation.Horizontal;
    radListView1.Items.Add("Item 1");
    radListView1.Items.Add("Item 2");
    radListView1.Items.Add("Item 3");
    radListView1.Items.Add("Item 4");  
}

Workaround: set the ListViewElement.ViewElement.Orientation property to Horizontal in the form's constructor.