To reproduce:
Me.RadDock1.DocumentTabsAlignment = Telerik.WinControls.UI.TabStripAlignment.Left
Me.RadDock1.DocumentTabsTextOrientation = Telerik.WinControls.UI.TabStripTextOrientation.Vertical
Please refer to the attached file.
Workaround:
Me.RadDock1.DocumentTabsAlignment = Telerik.WinControls.UI.TabStripAlignment.Left
Me.RadDock1.DocumentTabsTextOrientation = Telerik.WinControls.UI.TabStripTextOrientation.Vertical
For Each d As DocumentWindow In Me.RadDock1.DockWindows.DocumentWindows
    d.TabStripItem.AngleTransform = 180
Next
 
Or:
foreach (DocumentWindow d in this.radDock1.DockWindows.DocumentWindows)
{
    d.TabStripItem.FlipText = true ;
}