The only meaning of backlog, known to our dictionaries is a large log of wood forming the back of a fire on the hearth. But that is certainly not the meaning it is intended to convey in:
The most important step is to eliminate a very heavy backlog of orders on the manufacturers' books.
"Backlog" is perhaps more picturesque than "arrears" but it is not so intelligible, at any rate in this country. To us the G.P.O.'s "Waiting list of applicants " is clearer than the American Telephone Company's way of putting the same thing, "Backlog of held orders ".