I think the fact that "flat/corporate" design took over seemingly everywhere at once, in interfaces, designs, logos, etc is what really gets people. It all did "shift" as a culture because it really was a unified effort.

I think they visually represented a time when computing was pitched as fun and magical. Computing had a heavy emphasis on the joy and wonder of making & learning things, meeting people, etc. Flat design takes away from all of that, appealing to everyone by appealing to no one.