A page that ranked fourth for two years slips to eleventh. Nothing about it changed. No algorithm update landed. The backlink profile is intact. Before auditing the page itself, check what else on the site is now competing with it.
The usual culprit
Someone published something adjacent. A new blog post, a refreshed service page, a landing page for a campaign — and it targets close enough to the same intent that the search engine now has to choose between them. It often chooses the newer page, which is weaker, and both end up lower than the original was alone.
Diagnosing it
Export your pages and their target keywords and run the overlap check. Collisions show up as pairs. Where two pages target near-identical intent and one recently dropped, you have your explanation.
Deciding what to do
Consolidate when both pages serve the same intent — merge the better content into the stronger URL and redirect the other. Differentiate when the intents are genuinely distinct but the targeting drifted; retarget the newer page rather than deleting work.
Try it: Keyword Cannibalization Checker on SeoWolf's Notepad.