Probably from Ed Zercher. An image search will bring up his photo of doing it with a squat bar.
There's a photo of Paul Anderson doing the same, in a contraption that's even more like a smith machine than Zercher's.
As for why stuff like this is popular all of a sudden, I have no idea.
Probably Crossfitty something or other -> Zerchers -> photos of Zercher's barbell leg press -> someone writing an article about it -> ...