Gonna wade in with ideas rather than facts on this (lol)
Looking at it from a semi-scientific perspective ...
I think the effect is dependent on both how the surface coat is applied AND the environmental factors the piece experiences over a long time .. all metal expands and contracts with heat and cold , and whether the piece "alligators" or not would be an indicator of how well the elasticity of the topcoat ( paint or litho ) holds up over time ... if the topcoat is not 100% bonded to the entire area , over time the weaker attachment lines will have more stresses on them as they are no longer fully in contact with the underlying metal ( on a molecular level ) thus they crack minutely along weaker "fault lines" as they are pushed and pulled by the expansion and contraction , and over time the topcoat contracts away from the lightly fixed areas as it no longer has the bonds to keep it taut across the entire surface . the topcoat is still bonded to itself more than the metal or base-coat so even tho the coating doesn't flake off you get "Islands" of coating moving on the surface of the metal or base coat.
Most surface coatings tension as they dry or lose solvents, its why brush marks even out a little after drying etc ... Even when "dry" coatings continue to off-gas volatile's over a very long time ( in minute amounts ) and its often these volatile's that give coatings their flexibility.. which explains why alligatoring takes a long time and is associated with decent age . The effect could also take place WITHIN the coating itself if the "picture" part is a separate identity from the overall composition.
Litho in particular is several layers of pigment bearing substrate ( ink) applied over each other ... over time each colour layer may also minutely expand and contract at different rates .. the Green over Yellow of Dons AF tender seems to be that way .. the green top ink has moved and contracted over time more than the yellow base ink which has remained tightly affixed to the metal , thus the green ink "islands" have formed and contracted over the years ?
Its what commercial orange peel paints and processes rely on , also Hammerite etc ... the Hammerite is a tough intact coating , but as it dries the pigment components are less attached to the binding matter and they shift to create the distinctive Hammered Look .
The clever bit with them is they keep the outer and inner surfaces of the coating attached to whatever you are painting , allow the alligatoring of the pigment to take place then bind it in super tough top and bottom clear layers to provide a durable overall finish ... they achieve intentionally the same process in minutes rather than the years it takes our toys to 
