The Search Engine node is automatically placed in search-only mode when the location of the search index is moved or when the index size of the Search Engine reaches 50% of the available disk space.
When a Search Engine node is in search-only mode, data that was content indexed by the Search Engine node can be searched, but no new data is pushed to the Search Engine node for content indexing.
You can manually place the Search Engine node in search-only mode by running the DisableSearchEngineNode qscript.