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From my experience, this most often happens with classic local business sites strongly focussed on one product or service.
Let’s take a plumber. Mainpage is mainly about plumbing, then there are service pages under one main category page which is “Plumbing”. Mainpage has less content, but is stronger. Plumbing subpage has more content, but is weaker. Basically two pages competing for a ranking for the same term or what you could call “near duplicate content”.
There are two ways to solve this: Separating or merging. You either use really different titles and make sure the content is different enough or you merge them into one and do a 301 for the one you don’t want to keep.
One really huge sites with some pages constantly cannibalizing and switching positions, this is far less obvious of course. The best way to avoid it in the first place is proper documentation, whenever you want to publish a new piece, you check if this topic wasn’t already (partly) covered.
