Parenting is a harrowing experience. You try to maintain a sense of decorum as your son decides he would rather share the dog's food than eat his own, or your daughter decides to to ride her Big Wheel down the street wearing nothing on but a smile. By comparison, an inexplicable obsession with garbage trucks is not that big a deal. But it does make one wonder.
Kerry Novick, a psychoanalyst writing for AnnArbor.com, takes a stab at explaining the phenomenon.