Google has launched a new format for the sitelinks on the mobile search results. Shree Vaidya spotted the change, and I am able to replicate the new look for sitelinks.
Google Sitelinks are the links under a search result snippet that add additional navigation links to important, often relevant, pages on the website.
Here is a picture of the new look, where you can expand a category of the site and then see subsections within the link:
As you can see, when I expand SEO, it shows me four additional links, including the popular Google Algorithm Updates category. The results are not perfect, but Google may be testing this to see the level of interaction with their users.
Google tested these types of sub-sitelinks in 2013.