I'd say it started with the mobile web. Lots of space and simplistic interfaces make a lot of sense when you have a small screen and fat fingers. It is terrible on a desktop but since people want a unified experience, to keep things familiar to the user and to simplify development, you get the least common denominatior.

I'd say that the "flat" trend was started by Microsoft with Windows Mobile and Windows 8. Apple and Google followed (for once Apple didn't start the trend). I liked it at the time, maybe for the novelty, but it quickly faded. Still I understand why people may like it.