You say you were not aware that they had ever really gone out of fashion?
Well, that's a good point.
But you have to understand. The key to trend-spotting is a willingness to simply make declarations and then wait.
A Margaret Atwood novel comes to mind. The lead character was a writer who covered fashion for magazines. Tired of her job, she got lazy and started making things up.
One of the “trends” she reported had to do with women wearing vintage bathtub-stopper chains as necklaces. And sure enough, after her story appeared in a magazine, the writer started seeing that exact look on women she passed on the sidewalk.
OK, that's not exactly the same as the plaid shirts thing, because you actually do see those in real life. But if enough people declare that they are making a comeback, I'd bet we would see even more.