Planet13 - 12:21 am on May 24, 2012 (gmt 0)
In my case, I think it is definitely a combination of over use of the keyword phrase on the page, and the use of the keyword in INTERNAL links from other pages on the site.
The page used to rank about number 9, but then dropped out of the SERPs and instead the home page is ranking in its place.
That is for the plural version of the keyword. For the singular version of the keyword, it still ranks in the same place.
All the internal links that use the keyword text used the plural version.