To reproduce:
- Try to set the  alignment:
radPopupEditor1.PopupEditorElement.PopupContainerForm.HorizontalPopupAlignment = HorizontalPopupAlignment.RightToRight;
radPopupEditor1.Popup.HorizontalPopupAlignment = HorizontalPopupAlignment.RightToRight;

- The property is reset when the popup is opened.

Workaround:
void radPopupEditor1_PopupOpening(object sender, CancelEventArgs e)
{
    RadPopupOpeningEventArgs args = e as RadPopupOpeningEventArgs;
    int width = radPopupContainer1.Width;

    args.CustomLocation = new Point(args.CustomLocation.X - width + radPopupEditor1.Width, args.CustomLocation.Y);
}