The person who didn't reverse is supposed to wave politely to the person who did reverse - that's the usual behaviour in Scotland.
Also its sometimes not just who is nearest who reverses back - sometimes things like corners, size of vehicle, if anyone has a trailer come into the decision making.
I assumed "don't be a jerk, wave to the other guy" and "don't make the lorry back up hill and around a curve" were given. :)
Of all the places I've driven, I've found Scotland (Perthshire and into the Highlands in particular) to be the most sane. Polite, little drama, and most follow the rules to a T.
Also its sometimes not just who is nearest who reverses back - sometimes things like corners, size of vehicle, if anyone has a trailer come into the decision making.