ecnext - 2:49 pm on Apr 21, 2010 (gmt 0)
What if someone does a WebmasterWorld search, then links to one of the search results that includes the highlighted term in the URL? Is that the URL pattern you're seeing requested by Googlebot?
You might be able to find some of these links if you download and scan through the "Links to your site" from webmaster tools.
This could be a good case for the canonical tag.