I worked as an outside UX/UI consultant with Omniture (acq. by Adobe) in 2007 during which I was tasked with creating more than 9,000 individual icons to create continuity and flow for their next release. I deviated from the current 'candy' design trend and subsequently influenced the design of what would become the first major 'flat' application interface.
Because Omniture was the most future forward tool in online marketing at the time, this introduction to the style was viewed as /the/ new emerging trend. Subconsciously people build products based on the influence of both their tools and environment, so Omniture's customer's products began incorporating little details they saw in the Omniture interface.
This new style slowly influenced the look and feel of everything from websites to iOS. I never would've envisioned that this 'look' would have taken hold like it did, but it is amazing to see it continue to evolve through other amazing designer's contributions.