I was just talking to a friend about a book she was reading and she said it goes back and forth between time periods and all I said was, “Of course it does.” Some things sell, I guess.
As far as I can tell, this trend originates with AS Byatt's Possession. I read it soon after it came out, and I remember the dual timelines seeming fresh and intriguing, but very few writers can pull it off as she did, and it's become just another standard narrative structure.