To reproduce:
Add a TabControl and two tabs. In the second tab add a RadGridView and in the Load event of the form set the SplitMode property of the grid. You will notice that it will not have effect until grouping or other similar action has been performed.

Workaround:
private void Form1_Load(object sender, EventArgs e)
{
    DataTable dt = new DataTable();
    dt.Columns.Add("column1");
    dt.Columns.Add("column2");

    RadGridView1.DataSource = dt;

    this.RadGridView1.VisibleChanged += Visible_Changed;

}

private void Visible_Changed(object sender, EventArgs e)
{
    if (this.RadGridView1.Visible & !this.RadGridView1.SplitMode.Equals(Telerik.WinControls.UI.RadGridViewSplitMode.Horizontal)) {
        RadGridView1.SplitMode = Telerik.WinControls.UI.RadGridViewSplitMode.Horizontal;
    }
}