If you think this was just another fad, then you are mistaken. Flat design is minimalist design on steroids. It’s about keeping it simple, clean and modern which are words you often hear out of most clients’ mouths.
The reason why everybody is using flat design is because it makes sense. We have less clutter, more use of white space and overall a better user experience. It forces designers to achieve more by using less and for that, I consider it the number one trend for 2014.