It's an interesting trend that I have seen among my company's user base. It could be real, but it could also be an increase in false positives.
A possible explanation for the false positives:
Many large companies are seeing more and more machines moving to IE11, but have websites/tools that require IE8. Ideally each web site should explicitly declare compatibility/standard modes they require in their HTML. But for older enterprise software, this is not an option, so companies take the 'easy' approach of enabling compatibility mode for all sites and/or all intranet sites. A better approach when you can't edit the html of the web app is to use the group policy editor and set compatibility mode on a URL by URL basis. But many companies take the easier route. This is my theory that may explain the false positives.
If they aren't doing so, tools that aggregate browser usage should be doing additional analysis on the user agent string to get a better sense of the 'true' IE version. Perhaps using methods like ie-truth [1]. When I see trends like increased IE8 usage, it doesn't make sense to me... so I have doubts that these tools are testing the browser type accurately.
[1] https://github.com/Gavin-Paolucci-Kleinow/ie-truth