Paginated pages compete with the main category page because they share the category’s topic and can rank for the category term themselves, which splits the signals between them. Page 2 and page 3 of a category are still about the same subject as the category page, made of the same kind of content, often with similar on-page text and the same internal links pointing at the topic. When several near-equivalent URLs all signal relevance for one query, Google has to choose among them, and the relevance, links, and authority that should consolidate onto one strong page get spread across the set. This is observed behavior to watch for, not an inevitability, and it gets worse when the pages are mis-canonicalized.
The mechanism is shared topic without a clear hierarchy. If nothing tells Google which URL is the primary one for the category, the paginated pages look like competitors rather than continuations. Mis-canonicalization makes it sharper: when page 2 points its canonical at the category page (or at page 1) instead of at itself, you are telling Google those pages are duplicates of one another, which can suppress the deeper pages or confuse which one should rank. Either way, the signals that should stack behind the category page get divided, and a paginated page can surface for a query the category page should own.
The reason it is avoidable is that the competition comes from missing hierarchy, not from pagination existing. Pagination is a normal way to break a long list into pages. The problem appears only when the set has no clear primary and the canonicals do not reflect that each paginated page is its own distinct page in a sequence, not a duplicate of the category.
For your next setup, have each paginated page self-canonical (point its canonical tag at its own URL, since page 2 is genuinely a different page from page 1), and keep the category page as the clear primary for the category term through your internal linking and on-page focus. Then check in your reporting whether any paginated URL is ranking for the head category query, and if one is, treat it as a signal-split to resolve rather than a win.