I'd be more worried that the book wasn't keeping you engaged. Usually when my mind goes wandering it's a sign that the book is boring me (not always the book's fault, sometimes I can be hard to please). Such wanderings do sometimes spark very strange associations, though I don't recall any examples to offer as comparison (because they are usually quite fleeting thoughts).
While I was reading your post my mind did start thinking about Terry Pratchett's gnome books (The Gnome Trilogy / The Bromeliad Trilogy, depending on where you live) - because some of his gnomes ride geese. Which isn't to say your post was boring

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