As first spotted by Shree Vaidya over this past weekend, Google is testing expandable sitelinks on mobile search results.
Google tests new features quite often, so this expandable sitelinks rollout is neither a) surprising nor b) definitely permanent. From what I can tell so far, the expandable sitelinks only apply to organic results — though, should Google make these a long-term fixture, I would see no reason why the company wouldn’t eventually incorporate these into paid search as well (though, with Google’s “callouts” extensions, paid results don’t show sitelinks as often as before).
I was able to replicate Shree’s findings on my iPhone 6+, on both Chrome and Safari browsers.
Google first introduced sitelinks back in 2013. While site owners cannot designate which sitelinks appear in organic search results, they can demote algorithmically-decided ones in Webmaster Tools.