Coming from a base of long term E-commerce site my input may not matter but just some food for thought, if you kept the onsite or on page content exactly 100% the same and just changed the urls with the redirects without adding additional links to that specific page could one concur that this may be a result of over deep linking to that page or perhaps just the ending url tag? Did you 301 redirect the old pages to the new ones?
Yes, I 301'd the old pages to the new ones.
I'm pretty sure the results wasn't over deep-linking to that page, but unfortunately I can't prove it without going into details about the site, which I'd rather not.
What I can tell you is that NOTHING changed other than the URL, and the breadcrumbs for that url on the posts in that category.