There is some discussion of this here but I haven’t found a more comprehensive discussion of it.
With Material Design, objects that wish to convey being pressed appear to rise up towards the user, rather than get depressed away, as normal buttons do. (Example)
Why? As pointed out in the original link, it’s contrary to what we usually expect, and more importantly, it’s contrary to what happens in real life (which is what material design is supposed to represent)
On one of my own sites, at the time this was originally posted, I specifically designed the social media buttons in the footer (and most other buttons) to appear satisfyingly pressed when active