By default, DocumentPosition is associated to RadDocument, so the first and second relations are normal. When a RadDocument is set to RadRichTextBox.Document property, RadRichTextBox subscribes to its LayoutModeChanged event, so this explains the third relation. (When RadDocument is removed from RadRichTextBox.Document, the event handler is unsubscribed.)
Whether it's normal for this particular DocumentPosition to be GC root - I cannot tell for sure by just looking at the screenshot - maybe the screenshot was taken during a method execution and it's kept by a local variable in a method somewhere.
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