In Britain, 1/72 was an aircraft scale, as Tim says, and in Imperial or American measurements it makes sense: 1 inch = 6 feet. Again, as he says, 1/76 for military vehicles came into use because it fit the dominant British railway scale of OO-gauge — that’s why the only major manufacturers in 1/76 were British: Airfix and Matchbox, primarily. As I recall, 1/72 for vehicles came about when Esci (from Italy) decided to make small-scale vehicles, and fairly naturally picked the scale that they were already making aircraft models in.
1/32 was also a standard British scale, being used for large-scale vehicles and figures by Airfix in the 1970s. Not quite sure why they picked that number rather than, say, 1/36 (2 × 1/72), but they did. 1/35 is — by accounts — the result of Tamiya wanting to release a battery-powered Panther tank: to fit two D-type (I think) batteries side by side in the hull, it turned out it needed to be 1/35 scale. They then made their following military models to the same scale so they fit with the Panther, and soon the scale was so well-established that almost everybody else began using it too.