Whatever started it was not a smart idea. Humans have the ability to perceive depth from gradients and shadows very fast, it's dumb not to use it.
To me it seems like it really got rolling with Apple - Steve Jobs decision to stop doing the more richly detailed skeuomorphic designs. Which prompted flat redesigns in iOS 7 and OSX, and I think many developers followed suit to either look like today's Apple or not to be the nail to be hammered down in the iOS community. Also attributed is Microsoft Zune and Metro design work.<p>Historically flat design is a form of minimalist design articles point to it being rooted in Soviet design styles.