'"Back in the day" goes along with "Time was".'

But I never heard "back in the day" in my life until five or ten years ago. Then suddenly the kids were saying it all the time, then the adults took to saying it, and now I catch myself about to say it from time to time (it's so nonsensical it drives me up the wall). So back to my question: why did it suddenly become fashionable? Surely someone can cobble together a far-fetched theory ...

On to other news. Never heard "stay out of the trees" in Canada. "Round the bend" for crazy, especially in the sense of having gone mad with some obsession, is well-known in Canada.