my guess is that title has something to do with the pose... maybe. tried googling it and found a photo agency named maground so can't find any reference in english sadly
Edit: maybe the japanese title of this chapter will find something maybe?
Edit2: mago is grandkid or something similar so... guessing the title implies a grandchild scratching grandparent's back... makes sense with the content in the chapter but idk if it helps with the title. Anyways, I tried my best trying to solve the chapter title.
I don't know japanese much, but I remember reading that the japanese word for "backscratcher" can be translated as "grandchild's hand" or something in that line. Maybe it's related to that?