More than likely it was due to Norton scanning the site and finding "hits" on certain words on some of the pages. We used to have medical sites (like medical school on-line libraries) blocked at work because the filters saw certain anatomical names that were on the filters "hit list". A Community Board thread about pornography or child molesters could be even enough to trip a trigger.
It could also be due to "meta tags" that contain information about the web page. Meta tags are hidden from normal view and can only be seen when viewing the HTML source code for the page. For example, here's the keyword meta tag for the DIS's "Reply To Thread" page:
<meta name="keywords" content="disney world,walt disney world,orlando vacation,
disney cruise line,
disney cruise,disney hotels,disney resort,disney discount,disney world hotel,orlando theme park,disney tips,
disneyland,disney discussion,disney blog,disney message board" /> Search engines use these meta tags to catalog sites. I'm not making the claim that this is what happened here, but sometimes code can get reused from one developer's site to another and they forget to update the meta tags, so the users thinks the site is about one thing, and search engines and blocking software might think it's about something else. Again, I'm not claiming that's what happened here, but that's one scenario where an apparently benign site could get blocked.