From John (Webnauts):
Supplemental Pages
Common Causes of Supplemental Pages:
- Duplicated content;
- Too much content similarity;
- Pages with low or no content;
- Orphaned web pages. Pages that no one links to, including yourself;
- Error pages, if a site does not use If-Modified-Since, Last Modified and/or Expires rules;
- Poor website navigation;
- Pages due to canonicalization problems;
- Too low PageRank;
- Long URLs, especially with long parameters, starting with a question mark (?) and being separated with an ampersand (&) and are not rewriten;
- Suspicious pages for spamindexing, like non-unique and irrelevant to page content meta tags, or linking to bad neighborhoods, etc.
In this case I think it is back links as mentioned above. Sure not every page needs a back link to be out of the SI, but your main website better have plenty of quality back links so that when you link out to these sub-pages they don't fall into the SI.