A frustrating page falls over time because evidence accumulates. It can rank initially on relevance and authority, the signals that get a page into contention before many users have interacted with it, but as searchers arrive and consistently bounce back, linger less, or pick competitors, those behavioral patterns and the existence of pages that satisfy users better give Google reason to re-rate it down. The page wins the slot first and loses it later, not because the algorithm changed its mind on a whim but because the system gathered evidence that the page disappoints. This is observed behavior worth confirming in your own data, not a published guarantee.
The assumption to discard is that if a page ranks, its user experience no longer matters. That is true only for the moment of the initial ranking, which is largely a prediction. Relevance and authority are signals available before real searchers weigh in, so a frustrating page can clear that bar on topical fit and links alone and land on the first page. But the initial ranking is a bet that the page will satisfy the people who arrive, and that bet gets tested the moment traffic starts flowing.
What follows is a slow correction as evidence builds. Each searcher who returns to the results, abandons the page quickly, or chooses a competitor adds a data point, and over many visits the aggregate begins to contradict the initial prediction. Meanwhile competitors that genuinely satisfy users accumulate the opposite evidence. The system has more to go on than it did at first, and it re-rates accordingly, which is why the fall is delayed rather than immediate. The ranking was provisional all along; experience is what eventually confirms or revokes it.
For your own pages, do not treat reaching the first page as the finish line, treat it as the start of the test. Watch how real visitors behave once a page ranks, and fix the frustration (slow loads, hard-to-find answers, misleading promises, friction that sends people back to search) before competitors and accumulating evidence erode the position you just earned. The cheapest time to fix it is while you still hold the slot.