It's hard to say without looking at your page and the search term. Off the cuff my guess is its coincidence.
Google also sometimes "pulls" a URL temporarily from search results I'm guessing for re-evaluation if you make major changes to the URL (e.g. completely rewrite the TITLE tag, etc).
If your ranking drop isn't -300~900 range you can regain it; just do some smart, "natural-looking" link building (getting direct backlinks to the ranking page with varied anchor text leveraging links on your other domains instead of adding outbounds on this site of yours), and internal anchor text tweaks.
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