Google now preferring one page on site as others drop

Over the past few months Google has started to highly value one page on a site I maintain while the entry page continues to drop off steeply for pretty much every single other page. I've made no major changes and I haven't been able to find any suspicious third party activity thus far. I'm concerned this could effect the site in the long term because that page has little relevance compared to the rest of the site though it still serves a purpose for helping people. It could be any number of reasons, intentional or not such as putting too much emphasis on a page by a third party and then pulling-the-plug so-to-speak. I'm not really sure as I've never encountered this before. Suggestions on what I should look in to? The content on this site is all original and there is absolutely no advertising or any funny-business of any kind.

- John