I'm trying to determine right now whether it's been an issue of this particular post being the symptom of a broader discrimination against our site or whether there has been competition introduced for this page. All the peaks and valleys of the site's organic traffic are exactly the peaks and valleys of popularity for this post. Graphing other major (organic) landing pages for our site (the top three of which have much less traffic than this one stupid page) does not indicate that the other pages have been similarly affected — their popularity is far more undulating, and subject to far fewer crazy movements. So I'm pretty sure at this point that it's the one page.
And, yes, this particular blog post accounts for about 1/2 of our site's organic traffic. We've reduced the bounce rate on this blog post down to the low 80's, percentage wise, which I think is respectable for what the blog post is & it's relationship to the site and the site's purpose as a whole, which is commercial and not immensely related to the post's content.
I suppose that's a new question, isn't it? How much should we care about the fortunes of one page that has a high bounce rate? Obviously, we should reduce the bounce rate (and there are some things we haven't done yet to do that) but the nature of this particular post is just not a super strong match for the content and direction of our site. The bounce rate will always been fairly high, it's just the way it will always be. Yet it has so. much. traffic. Another site I work on has a similar page, similarly somewhat-tangential to the site's content: the "when to use spray foam insulation" page. Thus I always want to call these the "spray foam insulation pages."