I've seen this before. There are multiple reasons why something like this might happen. Is your subpage better targeted in terms of content, title tag ? Alternatively, is there even a little bit of the content of the sub-page duplicated on the home page ? My blog sometimes does this where the snippet from a blog post makes the /blog page rank higher than the blog post. Annoying !
In the prior case I'd drop a link to the home page on the sub page-but I'd also study why the sub page is grabbing the organics and perhaps change the home page to address this. In the latter case I'd make sure the page I want to rank has a decent amount of unique content that doesn't show up anywhere else on the site.