Popups (ImageMiniToolBar and SelectionMiniToolBar) stay on most top (act as if "Always on Top" is set to them) even if they had lost focus. 

Workaround: Attach to LostFocus or LostKeyboardFocus events and hide the toolbar in the handler:
this.radRichTextBox.LostFocus += (sender, e) =>
{
    this.radRichTextBox.SelectionMiniToolBar.Hide();
    this.radRichTextBox.ImageMiniToolBar.Hide();
};