One of the big NO of designing a website from an SEO point of view in the past was having hidden content inside button tabs.
From Google's perspective the user once clicking on your search result needs to have that content visible as soon as your page is accessed, this is still the case, the only difference is that now Google takes into consideration hidden content as being normal visible content.
This is great niche for all designers out there who always went into conflicts with SEOs over the structure and design of web pages.
Why does Google suddenly care about hidden content?
Because there is only so much content you can fill inside a mobile page! Tabs and hidden content is the only logical way to go on mobile pages, you can't fit everything you want as visible content, the page user will simply have to scroll a lot and when a user is getting annoyed the bounce rate goes up.
I think this is a natural evolution because of the recent statements from Google that mobile will become its main index.