Google deindexes pages during a core update because the update re-assesses quality and relevance across the whole site, and pages it now judges low-value or redundant can lose enough standing to drop out of the index entirely. Deindexing in this context is a re-rating outcome, not a random glitch or a bug, which is why treating it as a malfunction sends you looking in the wrong place. The page did not break; the bar moved and the page no longer cleared it.
A core update is, at its core, a broad re-evaluation of how Google judges content. It revisits its assessment of which pages best serve the queries they target, weighing quality, depth, usefulness, and relevance against everything else competing for the same intent. Most pages shift up or down in ranking as a result. But a page that the new assessment views as thin, outdated, duplicative, or simply outclassed by stronger alternatives can fall far enough that Google decides it is no longer worth keeping in the index. The drop from page one to page five and the drop out of the index entirely are points on the same scale; deindexing is just the far end of a downgrade.
This framing matters because it tells you the cause is your content’s standing under the new criteria, not an error to report. The pages that get dropped are usually the ones with the weakest case for inclusion to begin with: little unique value, heavy overlap with other pages, stale information, or content that no longer matches what searchers want. The update simply applied a stricter or differently weighted judgment, and those pages lost. This is observed behavior worth confirming against Google’s own guidance on core updates, since the specifics of each update vary.
To respond, look at what the update devalued on the dropped pages rather than waiting for them to return. Compare them honestly against the pages now ranking for the same queries, identify where they are thin, redundant, or outdated, and rebuild the unique value and relevance that gives Google a reason to index them again. Improving the content is the path back; treating it as a glitch is not.