This is from an old Japanese urban legend. A boy is called to come down stairs from his father, and the father briefly explains to his son that the mother is not "coming home."
Suddenly, the boy looks amused to this statement saying that he "sees mom right behind dad."
The original Japanese story uses the term "Onnbu," which means carrying someone on your back. So in the original, the boy asks his father "why are you "onnbu" mom?" (sorry, hard to translate)
Well, as you might guessed it by now, the father was the one who killed the mother. This is what makes the story a little freakier than just a missing mother after a big fight with the husband.