Interesting article. I hadn't thought about how flat design and skeuomorphism were precursors to material design.
I recently saw this video (https://www.youtube.com/watch?v=rrT6v5sOwJg
) about the making of material design and it makes a lot of sense. It's hard to call it a trend when they are making design choices based on what real material looks and feels like. I especially liked how they photographed and lit scenes with Google app icons cut out of paper - so the shadows would be real looking.
I think the ideas material design puts forth is so important now that we are interacting with the digital world with touch devices and not just a mouse and keyboard.
Although it's hard not to think there might have been an old graphic designer at Google who just couldn't wait to use a drop shadow again after flat design was here for so long.